Better fix for Record.remove() on detached group (stop the chain early)

Author: vicb

lib/change_detector/change_detector.dart

@@ -429,6 +429,7 @@ class Watch extends ChangeListener {
 
   /// Calls the [_reactionFn] when the observed [Record] value changes
   void _onChange(value, previous) {
+    assert(_triggerRecord._watchGroup.isAttached);
     super._onChange(value, previous);
     _reactionFn(value, previous);
   }
@@ -453,6 +454,8 @@ abstract class ChangeListener {
   /// current listener.
   void remove() {
     if (_triggerRecord != null) {
+      // The [WatchGroup] has been detached, no need to remove individual [Record] / [Watch]
+      if (!_triggerRecord._watchGroup.isAttached) return;
       if (identical(_triggerRecord._listenerHead, this)) {
         _triggerRecord._listenerHead = _listenerNext;
         if (_triggerRecord._listenerHead == null) {
@@ -714,8 +717,7 @@ class Record extends ChangeListener {
   }
 
   void remove() {
-    // Nothing to do as the [WatchGroup] has already been detached
-    if (!_watchGroup.isAttached) return;
+    assert(_watchGroup.isAttached);
 
     // Update watch counters
     if (isCollectionMode) {