This was caused by a missing refactor after the underlying data structure of the extra UVs was changed.
An additional test has been added to ensure this won't happen again.
There was an undocumented change in 5.0 where the shape keys now start with a weight of 1.0 instead of 0.0 when created.
I have added an assert in the shape key test to make sure that the values are set to `0.0`.
This also fixes the assertion in the poll function (asserts should never be in poll functions).
This was caused by a regression caused by 29831d7f09.
The test for importing extra UVs has been updated to check that the data is different between the different UV layers.
