PiP: Null-check menu view when dispatching touch events to the menu.

Although this should not happen, it is possible that sometimes touch
events can be dispatched to the menu controller when the menu is not
around (SystemUI crash, perhaps).

Bug: 170707099
Test: None
Change-Id: I0d0c9826b10bf9071902075f0ad51051a5009d17
1 file changed