Ran into this problem when making mods to ForestFire.
I simplified the issue and re-created the problem as follows:
1) Open a new project
2) "make a block" ("for this class only ...")
3) Save the project
4) Try and re-open the saved project
Note: The error does NOT occur if you create a block "for all objects ..."
Defined Blocks cause exception when trying to re-open project [FIXED]
Moderator: MSandro
Re: Defined Blocks cause exception when trying to re-open project
See fixed in V51 :)
Cheers
Cheers