A prior patch made the button to show/hide a VST check the current
status of a loaded VST. This lead to situation where upon first
selection a VST might be loaded but the button's visibility is not
changed.
This change will reevaluate the status of a loaded VST upon selection
in the drop-down menu and trigger button visibility accordingly.
This changes the buttons to better reflect the state of the filter:
* When no VST is loaded, hide both buttons
* Check if VST is loaded before changing visibility
* Check if VST is loaded and editor is open before changing visibility
Per OBS API documentation, `get_properties` can be used to get the
properties of a source, but also of a source type. The latter would
pass a NULL pointer for a given source.
This adds the necessary change to avoid crashing OBS by passing such
a null pointer.
