removeGroup method Null safety
- String groupKey,
 - dynamic groupID
 
Remove a group from this user's membership for a particular group key
groupKeyThe property name associated with this group type (must already have been set up).groupIDThe group value to remove.
Implementation
void removeGroup(String groupKey, dynamic groupID) {
  if (_MixpanelHelper.isValidString(groupKey)) {
    _channel.invokeMethod<void>('removeGroup',
        <String, dynamic>{'groupKey': groupKey, 'groupID': groupID});
  } else {
    developer.log('`removeGroup` failed: groupKey cannot be blank',
        name: 'Mixpanel');
  }
}