@Test public void testClearBackStackOnEmptyBackStack() {

	// Arrange:
	final FragmentManager mockManager = mock(FragmentManager.class);
	final FragmentController controller = new FragmentController(mockManager);
	when(mockManager.getBackStackEntryCount()).thenReturn(0);
	// Act:
	controller.clearBackStack();
	// Assert:
	verify(mockManager, times(2)).getBackStackEntryCount();
}