Blender Git Loki

Fix T90341: Crash opening 2.4 file with File Browser editor open

The file selection parameters (e.g. `context.space_data.params`) are
expected to be unset in certain cases. Reading 2.4 files seems to be one
of them. Hence, code using it should check it's set first. Also added an
assert to a File Browser UI template printing a message when the
parameters are unset, to help debugging the issue.