Add null check for WindowInfo

The WindowInfo may be null if the target window can't be found in
current mWindowInfoById. Add null guard to prevent from NPE here.

Bug: 116652152
Test: atest CtsAccessibilityServiceTestCases
Test: atest CtsAccessibilityTestCases
Change-Id: Iaf2446e66420289be4154e90fbe213bbca9fd41f