My code was working fine but now I'm unable to see the Profile page as it generates an error and I'm unable to identify, even though I tried to return a Container() at the end of buildProfileButton() but it did not solve the issue as I updated the code of that function later on.

class Profile extends StatefulWidget {
  final String profileId;

  Profile({this.profileId});

  @override
  _ProfileState createState() => _ProfileState();
}

class _ProfileState extends State<Profile> {
  final String currentUserId = currentUser?.id;
  bool isLoading = false;
  int postCount = 0;
  bool isFollowing = false;
  int followerCount = 0;
  int followingCount = 0;
  List<Post> posts = [];

  @override
  void initState() {
    super.initState();
    getProfilePosts();
    getFollowers();
    getFollowing();
    checkIfFollowing();
  }

  checkIfFollowing() async {
    DocumentSnapshot doc = await followersRef
        .document(widget.profileId)
        .collection('userFollowers')
        .document(currentUserId)
        .get();
    setState(() {
      isFollowing = doc.exists;
    });
  }

  getFollowers() async {
    QuerySnapshot snapshot = await followersRef
        .document(widget.profileId)
        .collection('userFollowers')
        .getDocuments();
    setState(() {
      followerCount = snapshot.documents.length;
    });
  }

  getFollowing() async {
    QuerySnapshot snapshot = await followingRef
        .document(widget.profileId)
        .collection('userFollowing')
        .getDocuments();
    setState(() {
      followingCount = snapshot.documents.length;
    });
  }

  getProfilePosts() async {
    setState(() {
      isLoading = true;
    });
    QuerySnapshot snapshot = await postsRef
        .document(widget.profileId)
        .collection('userPosts')
        .orderBy('rating', descending: true)
        .getDocuments();

    setState(() {
      isLoading = false;
      postCount = snapshot.documents.length;
      posts = snapshot.documents.map((doc) => Post.fromDocument(doc)).toList();
    });
  }

  Column buildCountColumn(String label, int count) {
    return Column(
      mainAxisSize: MainAxisSize.min,
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        Text(
          count.toString(),
          style: TextStyle(fontSize: 22.0, fontWeight: FontWeight.bold),
        ),
        Container(
          margin: EdgeInsets.only(top: 4.0),
          child: Text(
            label,
            style: TextStyle(
              color: Colors.black,
              fontSize: 15.0,
              fontWeight: FontWeight.w400,
            ),
          ),
        ),
      ],
    );
  }

  editProfile() {
    Navigator.push(
        context,
        MaterialPageRoute(
            builder: (context) => EditProfile(currentUserId: currentUserId)));
  }

  Container buildButton({String text, Function function}) {
    return Container(
      padding: EdgeInsets.only(top: 2.0),
      child: FlatButton(
        onPressed: function,
        child: Container(
          width: 210.0,
          height: 27.0,
          child: Text(
            text,
            style: TextStyle(
              color:
                  isFollowing ? Colors.white : Theme.of(context).primaryColor,
              fontWeight: FontWeight.bold,
            ),
          ),
          alignment: Alignment.center,
          decoration: BoxDecoration(
            color: isFollowing ? Theme.of(context).primaryColor : Colors.white,
            border: Border.all(
              color: Theme.of(context).primaryColor,
            ),
            borderRadius: BorderRadius.circular(5.0),
          ),
        ),
      ),
    );
  }

  buildProfileButton() {
    bool isProfileOwner = currentUserId == widget.profileId;
    if (isProfileOwner) {
      return buildButton(
        text: "Edit Profile",
        function: editProfile,
      );
    } else if (isFollowing) {
      return buildButton(
        text: "Unfollow",
        function: handleUnfollowUser,
      );
    } else if (!isFollowing) {
      return buildButton(
        text: "Follow",
        function: handleFollowUser,
      );
    }
  }

  handleUnfollowUser() {
    setState(() {
      isFollowing = false;
    });
    followersRef
        .document(widget.profileId)
        .collection('userFollowers')
        .document(currentUserId)
        .get()
        .then((doc) {
      if (doc.exists) {
        doc.reference.delete();
      }
    });

    followingRef
        .document(currentUserId)
        .collection('userFollowing')
        .document(widget.profileId)
        .get()
        .then((doc) {
      if (doc.exists) {
        doc.reference.delete();
      }
    });

    activityFeedRef
        .document(widget.profileId)
        .collection('feedItems')
        .document(currentUserId)
        .get()
        .then((doc) {
      if (doc.exists) {
        doc.reference.delete();
      }
    });
  }

  handleFollowUser() {
    setState(() {
      isFollowing = true;
    });
    followersRef
        .document(widget.profileId)
        .collection('userFollowers')
        .document(currentUserId)
        .setData({});

    followingRef
        .document(currentUserId)
        .collection('userFollowing')
        .document(widget.profileId)
        .setData({});

    activityFeedRef
        .document(widget.profileId)
        .collection('feedItems')
        .document(currentUserId)
        .setData({
      "type": "follow",
      "ownerId": widget.profileId,
      "username": currentUser.username,
      "userId": currentUserId,
      "userProfileImg": currentUser.photoUrl,
      "timestamp": timestamp,
    });
  }

  buildProfileHeader() {
    return FutureBuilder(
      future: usersRef.document(widget.profileId).get(),
      builder: (context, snapshot) {
        if (!snapshot.hasData) {
          return circularProgress();
        }
        User user = User.fromDocument(snapshot.data);
        return Padding(
          padding: EdgeInsets.all(16.0),
          child: Column(
            children: <Widget>[
              Row(
                children: <Widget>[
                  CircleAvatar(
                    radius: 40.0,
                    backgroundColor: Colors.grey,
                    backgroundImage: CachedNetworkImageProvider(user.photoUrl),
                  ),
                  Expanded(
                    flex: 1,
                    child: Column(
                      children: <Widget>[
                        Row(
                          mainAxisSize: MainAxisSize.max,
                          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                          children: <Widget>[
                            buildCountColumn("Videos", postCount),
                            buildCountColumn("Followers", followerCount),
                            buildCountColumn("Following", followingCount),
                          ],
                        ),
                        Row(
                          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                          children: <Widget>[buildProfileButton()],
                        ),
                      ],
                    ),
                  ),
                ],
              ),
              Container(
                alignment: Alignment.centerLeft,
                padding: EdgeInsets.only(top: 12.0),
                child: Text(
                  user.username,
                  style: TextStyle(
                    fontWeight: FontWeight.bold,
                    fontSize: 16.0,
                  ),
                ),
              ),
              Container(
                alignment: Alignment.centerLeft,
                padding: EdgeInsets.only(top: 4.0),
                child: Text(
                  user.displayName,
                  style: TextStyle(
                    fontWeight: FontWeight.bold,
                  ),
                ),
              ),
              Container(
                alignment: Alignment.centerLeft,
                padding: EdgeInsets.only(top: 2.0),
                child: Text(
                  user.bio,
                ),
              ),
            ],
          ),
        );
      },
    );
  }

  buildProfilePosts() {
    if (isLoading) {
      return circularProgress();
    } else if (posts.isEmpty) {
      return Container(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            SvgPicture.asset('assets/images/no_content.svg', height: 260.0),
            Padding(
              padding: EdgeInsets.only(top: 20.0),
              child: Text(
                "No Posts",
                style: TextStyle(
                    color: Colors.redAccent,
                    fontSize: 40.0,
                    fontWeight: FontWeight.bold),
              ),
            ),
          ],
        ),
      );
    }

    return Column(
      children: posts,
    );
  }

  buildTogglePostOrientation() {
    return Row(
      mainAxisAlignment: MainAxisAlignment.spaceEvenly,
      children: <Widget>[
        IconButton(
          icon: Icon(Icons.videocam,
              size: 30.0, color: Theme.of(context).primaryColor),
          onPressed: null,
        ),
      ],
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: header(context, titleText: "PROFILE"),
      body: ListView(
        children: <Widget>[
          buildProfileHeader(),
          Divider(),
          buildTogglePostOrientation(),
          Divider(
            height: 0.0,
          ),
          buildProfilePosts(),
        ],
      ),
    );
  }
}

The debug console says that: A non-null String must be provided to a Text widget. 'package:flutter/src/widgets/text.dart': Failed assertion: line 285 pos 10: 'data != null'

The relevant error-causing widget was FutureBuilder


Solution 1: Ali Qanbari

Instead of writing Text(description), write Text(description ?? '') so the Text widget will be empty if description is null.

When using the property of an object, instead of writing Text(user.bio) write Text(user?.bio ?? '') for the Text widget to get an empty string if user or user.bio is null.

You can Also make sure that the strings your using in the Text widget are never null.

Not related to the question but I see that you use a lot of functions to build your UI, I suggest that you turn each of them to a separate widget to improve your app's performance. see this post to learn more.