PiP: Safeguard in calling on menu view in case it's not created.

In case SystemUI crashes, there is a possibility that we can call on
showMenu() upon SysUI restart without the menu instance created. We
should at least not crash SysUI again for this.

Bug: 173683353
Test: None
Change-Id: I67c11ffaf9f7e6972cc4b005f07f8fc6897ac5c1