Attempt to fix NPE in keyguard

This attempts to fix a bug where it looks like we have null content in
one of the widget pages in keyguard.

Based on the bug description, it's likely the transport control was just
removed from the view hierarchy.  Not sure how the content can be null
in this case, but this will hopefully prevent the crash and give us some
insight into what caused it.

Fixes bug 8886916

Change-Id: I22c26c49f22fa5b06987e8199070a9aaead2ff8a