5.0.4 Project Load Fix

[krzywon]

This fixes a project load issue noted by @RichardHeenan in a comment in 1689. This fixes the issue with loading projects with 3 (or more) fits. I am working on the errors thrown in another branch.

[rozyczko]

Good code.
The only problematic thing I can see is if you had empty self.tabs list. This would throw a ValueError from max. However, self.tabs is never empty, as deleting the last tab always creates a new tab in the perspective with incremented tab_id

[krzywon]

Good catch, @rozyczko. I'll add a default value to max for completeness. Would 0 be an appropriate value? An empty list implies no tabs exist and self.maxIndex would be set to 1.