Bug fix a race condition when Scope(useNullFieldInitialization = true) is set.

[rysh88]

A race condition occurs when Scope(useNullFieldInitialization = true) is set. This fix ensures the correct variable is checked so the correct value is returned.

current code

Activity activity() {
    Object _activity = activity;
        if (_activity == null) {
          synchronized (this) {
            if (activity == null) { // race condition here. The activity can be non-null and return a null _activity
              _activity = appCompatActivity();
              if (_activity == null) {
                throw new NullPointerException("Factory method cannot return null");
              }
              activity = _activity;
            }
          }
        }
        return (Activity) _activity;
  }

New Code

Activity activity() {
    Object _activity = activity;
        if (_activity == null) {
          synchronized (this) {
            _activity = activity; 
            if (_activity == null) { // local variable is checked instead of the global variable
              _activity = appCompatActivity();
              if (_activity == null) {
                throw new NullPointerException("Factory method cannot return null");
              }
              activity = _activity;
            }
          }
        }
        return (Activity) _activity;
  }