selectTree with MM table does not work without MM_opposite_field
When using TCA for a select and using the renderType='selectTree' it's not possible to edit fields, which have no configuration for MM_opposite_field.
A simple example is the categories field for pages and tt_content. If you unset the MM_opposite_field, it's not possible to edit the categories of pages and tt_content.
It's a strange bug in TYPO3\CMS\Core\Database\RelationHandler. I am not quite sure, but I believe the bug exists in the readMM-method. When you change the MM_match_fields to tablename instead of tablenames, it works again:
config type = select renderType = selectTree foreign_table = tx_example_related_data maxitems = 9999 MM = tx_example_mm MM_match_fields fieldname = field1 tablename = tx_example_product size = 10 treeConfig parentField = parent