An Error occurs when getting list data from another class in Flutter

We just refer the 'group' reference and after that we created an initial function for create an object that this reference refers. Here is our code snippet:

Class className{Group group,List<Group> membersList; void initializeData(){group = new Group(args);membersList.add(group}}

We call it from ListTile. Here is the callback scope:

ListTile{leading:className.membersList[index].name}

We got RangeError (index): Invalid value: Valid range value is empty : 0