filemanager: Add assertion checks to prevent dereferencing ptrs

* clang static analysis shows gaps where these calls could
  rereferencce pointers. Perform an assertion check.
filemanager: Add missing data assignment

* Amanas mentioned in IRC that it was likely
  missed after the gsettings migration.
Merge pull request #29 from kallisti5/master

filemanager: Add assertion checks to prevent dereferencing ptrs