stackFilter property Null safety

A callback which filters the stack trace. Receives an iterable of strings representing the frames encoded in the way that StackTrace.toString() provides. Should return an iterable of lines to output for the stack.

If this is not provided, then FlutterError.dumpErrorToConsole will use FlutterError.defaultStackFilter instead.

If the FlutterError.defaultStackFilter behavior is desired, then the callback should manually call that function. That function expects the incoming list to be in the StackTrace.toString() format. The output of that function, however, does not always follow this format.

This won't be called if stack is null.