Null check operator used on a null value

[lvyandev]

Describe the bug
A clear and concise description of what the bug is.

    await FilterListDialog.display<T>(
      this,
      listData: data,
      choiceChipLabel: (T? data) => titleBuilder?.call(data),
      validateSelectedItem: selectedDataValidator,
      onItemSearch: (T data, String query) =>
          searchComparator?.call(data, query) ??
          titleBuilder
              ?.call(data)
              ?.toLowerCase()
              .contains(query.toLowerCase()) ??
          false,
      hideSelectedTextCount: true,
      hideSearchField: true,
      allButtonText: MaterialLocalizations.of(this).selectAllButtonLabel,
      resetButtonText: 'reset'.translateAction(),
      applyButtonText: MaterialLocalizations.of(this).saveButtonLabel,
      onApplyButtonClick: (List<T>? data) => _completer.complete(data),
    );


======== Exception caught by gesture ===============================================================
The following _CastError was thrown while handling a gesture:
Null check operator used on a null value

When the exception was thrown, this was the stack: 
#0      FilterState.addSelectedItem (package:filter_list/src/state/filter_state.dart:46:18)
#1      ChoiceList._buildChoiceList.<anonymous closure>.<anonymous closure> (package:filter_list/src/widget/choice_list.dart:52:25)
#2      _RawChipState._handleTap (package:flutter/src/material/chip.dart:1716:24)
#3      _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:989:21)
#4      GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:198:24)
#5      TapGestureRecognizer.handleTapUp (package:flutter/src/gestures/tap.dart:608:11)
#6      BaseTapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:296:5)
#7      BaseTapGestureRecognizer.acceptGesture (package:flutter/src/gestures/tap.dart:267:7)
#8      GestureArenaManager.sweep (package:flutter/src/gestures/arena.dart:157:27)
#9      GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:443:20)
#10     GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:419:22)
#11     RendererBinding.dispatchEvent (package:flutter/src/rendering/binding.dart:322:11)
#12     GestureBinding._handlePointerEventImmediately (package:flutter/src/gestures/binding.dart:374:7)
#13     GestureBinding.handlePointerEvent (package:flutter/src/gestures/binding.dart:338:5)
#14     GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:296:7)
#15     GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:279:7)
#19     _invoke1 (dart:ui/hooks.dart:170:10)
#20     PlatformDispatcher._dispatchPointerDataPacket (dart:ui/platform_dispatcher.dart:331:7)
#21     _dispatchPointerDataPacket (dart:ui/hooks.dart:94:31)
(elided 3 frames from dart:async)
Handler: "onTap"
Recognizer: TapGestureRecognizer#f56cc
  debugOwner: GestureDetector
  state: ready
  won arena
  finalPosition: Offset(153.4, 286.0)
  finalLocalPosition: Offset(34.2, 16.0)
  button: 1
  sent tap down
====================================================================================================

To Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior
A clear and concise description of what you expected to happen.

 // Add item in to selected list
  void addSelectedItem(K item) {
    _selctedItems!.add(item); // _selctedItems wasn't initialized

    notifyListeners();
  }

  // Remove item from selected list
  void removeSelectedItem(K item) {
    _selctedItems!.remove(item);

    notifyListeners();
  }

Screenshots
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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.