local variable 'mouse_modifiers' referenced before assignment

Since upgrading from prompt-toolkit==3.0.19 to prompt-toolkit==3.0.28, I started seeing this issue pretty frequently.

  File "/usr/lib/python3.9/asyncio/events.py", line 80, in _run
    self._context.run(self._callback, *self._args)
  File "/root/.local/lib/python3.9/site-packages/prompt_toolkit/application/application.py", line 711, in read_from_input
    self.key_processor.process_keys()
  File "/root/.local/lib/python3.9/site-packages/prompt_toolkit/key_binding/key_processor.py", line 270, in process_keys
    self._process_coroutine.send(key_press)
  File "/root/.local/lib/python3.9/site-packages/prompt_toolkit/key_binding/key_processor.py", line 185, in _process
    self._call_handler(matches[-1], key_sequence=buffer[:])
  File "/root/.local/lib/python3.9/site-packages/prompt_toolkit/key_binding/key_processor.py", line 320, in _call_handler
    handler.call(event)
  File "/root/.local/lib/python3.9/site-packages/prompt_toolkit/key_binding/key_bindings.py", line 124, in call
    result = self.handler(event)
  File "/root/.local/lib/python3.9/site-packages/prompt_toolkit/key_binding/bindings/mouse.py", line 279, in _
    modifiers=mouse_modifiers,

Exception local variable 'mouse_modifiers' referenced before assignment
Press ENTER to continue...```

Start in prompt_toolkit/key_binding/bindings/mouse.py at the handler location shown in the traceback, and compare the mouse-event paths between prompt-toolkit 3.0.19 and 3.0.28. Reproduce the reported event-loop failure if possible; done means the local-variable exception no longer occurs for the affected mouse input.