'read_characters' referenced before assignment

[edgarcosta]

[I 19:33:01.708 NotebookApp] Starting buffering for 5aeb0f7b-2c59-430d-80c2-8e4fc8f4c960:e2c8f49b69a24aa784d54cb71c152110
[IPKernelApp] ERROR | Exception in message handler:
Traceback (most recent call last):
  File "/home/sage/sage-9.4/local/lib/python3.9/site-packages/ipykernel/kernelbase.py", line 268, in dispatch_shell
    yield gen.maybe_future(handler(stream, idents, msg))
  File "/home/sage/sage-9.4/local/lib/python3.9/site-packages/tornado/gen.py", line 735, in run
    value = future.result()
  File "/home/sage/sage-9.4/local/lib/python3.9/site-packages/tornado/gen.py", line 209, in wrapper
    yielded = next(result)
  File "/home/sage/sage-9.4/local/lib/python3.9/site-packages/ipykernel/kernelbase.py", line 543, in execute_request
    self.do_execute(
  File "/home/edgarcosta/.sage/local/lib/python3.9/site-packages/magma_kernel/kernel.py", line 210, in do_execute
    "text": self.child.before[read_characters:] + append_to_output,
UnboundLocalError: local variable 'read_characters' referenced before assignment