Right now a missing referenced plugin (or any other missing class) causes a sheet to fail to load correctly and appear blank. This must be fixed before plugins can be installed, deinstalled and refactored. We can provide mock interfaces for iPlugin (and quite possibly DefaultOverrides? as well) that will allow the sheet to load, but we need to make sure that those missing classes remain referenced on subsequent saves as well.
Additionally, we need at least a popup saying "missing plugin". It would also be nice to be able to send us to a plugin registry download page or something.
Alternatively, we sould do a regexp based sweep over the file first, to warn the develoepr that it isn't going to load, and give them an opportunity to download and install the correct plugin ?