What is id_data.keys() and how to prevent IDPropertyGroup change size during iteration?

I'm trying to revive an old blender add-on for personal use, all fixes were successful so far, but I stumbled upon this error:

for prop in o.id_data.keys(): RuntimeError:IDPropertyGroup changed size during iteration

and I don't understand what happens here. The code is supposed to clear a selected mesh from all properties/keys, but it stops since for got shorter and I can't substitute it for anything that works. I can't find what id_data.keys() is or does.

class SIMGEOM_OT_make_morph(bpy.types.Operator): """Give the selected objects data for a Morph GEOM""" bl_idname = "simgeom.make_morph" bl_label = "Make Morph" def execute(self, context): # Selected mesh objects selected = [ o for o in bpy.context.scene.objects if o.select_get() and o.type == 'MESH' ] # Must have 2 or more meshes selected of which one must be the active object if len(selected) == 0: self.report({'ERROR'}, "You must select 1 or more meshes to turn into morphs.") return {'CANCELLED'} n_copied = 0 for o in selected: if o.get('__GEOM_MORPH__') != None: continue # Clear old properties for prop in o.id_data.keys(): del o[prop] o['__GEOM_MORPH__'] = 1 n_copied += 1 self.report({'INFO'}, f"Turned {n_copied} objects into morphs.") return {'FINISHED'}

What do I need to change to make it work again?

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