fix nullify view on unmount

lib/android/app/src/main/java/com/reactnativenavigation/viewcontrollers/ViewController.java

 	}
 
 	public void onDestroy() {
-		if (view != null) {
-			view.getViewTreeObserver().removeOnGlobalLayoutListener(this);
-		}
 		if (isShown) {
 			isShown = false;
 			onDisappear();
 		}
+		if (view != null) {
+			view.getViewTreeObserver().removeOnGlobalLayoutListener(this);
+			view = null;
+		}
 	}
 
 	@Override

lib/android/app/src/test/java/com/reactnativenavigation/viewcontrollers/ViewControllerTest.java

 import org.junit.Test;
 import org.robolectric.Shadows;
 
+import java.lang.reflect.Field;
+
 import static org.assertj.core.api.Java6Assertions.assertThat;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.spy;
 		view.getViewTreeObserver().dispatchOnGlobalLayout();
 		verify(spy, times(0)).onAppear();
 		verify(spy, times(0)).onDisappear();
+
+		Field field = ViewController.class.getDeclaredField("view");
+		field.setAccessible(true);
+		assertThat(field.get(spy)).isNull();
 	}
 
 	@Test