Created attachment 245743 [details] [review]
Proposed patch

Applications have no way of finding out if a session manager proxy was successfully created in gtk_application_startup_session_dbus(), so it's not appropriate for certain public GtkApplication functions to be asserting the presence of a session manager proxy as if it were a programmer error.

This affects:

   gtk_application_inhibit()
   gtk_application_is_inhibited()

If sm_proxy is NULL, the function should just return silently.

In the case of gtk_application_uninhibit(), the application should only be calling this if it obtained a valid cookie, which implies the presence of a session manager proxy.  I noted that with a comment.