Guard against non-object returns from File::getHander()

MediaHandler::getHandler() can return false when a handler cannot be
determined for the given file's derived mime type. This change adds
guards to invocations that I could find that did not properly check for
this potential return result.

Bug: 53820
Change-Id: I8c0165311cc75f9920ac30ce2b38ccd207439198
Merge "Guard against non-object returns from File::getHander()"