Debugger raises a KeyError referencing matplotlib.pyplot

Issue Type: Bug

Backend TkAgg is interactive backend. Turning interactive mode on.
Traceback (most recent call last):
  File "c:\Users\ratanase\.vscode\extensions\ms-python.python-2018.9.0\pythonFiles\experimental\ptvsd\ptvsd\_vendored\pydevd\_pydevd_bundle\pydevd_frame.py", line 649, in trace_dispatch
    self.do_wait_suspend(thread, frame, event, arg)
  File "c:\Users\ratanase\.vscode\extensions\ms-python.python-2018.9.0\pythonFiles\experimental\ptvsd\ptvsd\_vendored\pydevd\_pydevd_bundle\pydevd_frame.py", line 134, in do_wait_suspend
    self._args[0].do_wait_suspend(*args, **kwargs)
  File "c:\Users\ratanase\.vscode\extensions\ms-python.python-2018.9.0\pythonFiles\experimental\ptvsd\ptvsd\_vendored\pydevd\pydevd.py", line 819, in do_wait_suspend
    self._activate_mpl_if_needed()
  File "c:\Users\ratanase\.vscode\extensions\ms-python.python-2018.9.0\pythonFiles\experimental\ptvsd\ptvsd\_vendored\pydevd\pydevd.py", line 442, in _activate_mpl_if_needed
    activate_function = self.mpl_modules_for_patching.pop(module)
KeyError: 'matplotlib.pyplot'
Traceback (most recent call last):
  File "c:\Users\ratanase\.vscode\extensions\ms-python.python-2018.9.0\pythonFiles\experimental\ptvsd\ptvsd\_vendored\pydevd\_pydevd_bundle\pydevd_trace_dispatch_regular.py", line 316, in __call__
    ).trace_dispatch(frame, event, arg)
  File "c:\Users\ratanase\.vscode\extensions\ms-python.python-2018.9.0\pythonFiles\experimental\ptvsd\ptvsd\_vendored\pydevd\_pydevd_bundle\pydevd_frame.py", line 649, in trace_dispatch
    self.do_wait_suspend(thread, frame, event, arg)
  File "c:\Users\ratanase\.vscode\extensions\ms-python.python-2018.9.0\pythonFiles\experimental\ptvsd\ptvsd\_vendored\pydevd\_pydevd_bundle\pydevd_frame.py", line 134, in do_wait_suspend
    self._args[0].do_wait_suspend(*args, **kwargs)
  File "c:\Users\ratanase\.vscode\extensions\ms-python.python-2018.9.0\pythonFiles\experimental\ptvsd\ptvsd\_vendored\pydevd\pydevd.py", line 819, in do_wait_suspend
    self._activate_mpl_if_needed()
  File "c:\Users\ratanase\.vscode\extensions\ms-python.python-2018.9.0\pythonFiles\experimental\ptvsd\ptvsd\_vendored\pydevd\pydevd.py", line 442, in _activate_mpl_if_needed
    activate_function = self.mpl_modules_for_patching.pop(module)
KeyError: 'matplotlib.pyplot'

VS Code version: Code 1.28.1 (3368db6750222d319c851f6d90eb619d886e08f5, 2018-10-11T18:13:53.910Z) OS version: Windows_NT x64 10.0.17763

System Info

Item	Value
CPUs	Intel® Core™ i7-7700 CPU @ 3.60GHz (8 x 3600)
GPU Status	2d_canvas: enabled checker_imaging: disabled_off flash_3d: enabled flash_stage3d: enabled flash_stage3d_baseline: enabled gpu_compositing: enabled multiple_raster_threads: enabled_on native_gpu_memory_buffers: disabled_software rasterization: enabled video_decode: enabled video_encode: enabled webgl: enabled webgl2: enabled
Memory (System)	47.79GB (29.09GB free)
Process Argv
Screen Reader	no
VM	0%

Extensions (19)

Extension	Author (truncated)	Version
gitdiffer	aag	0.0.3
scope-bar	amo	0.1.0
vscode-python-docstring	aza	0.0.5
jupyter	don	1.1.4
xml	Dot	2.3.2
gitlens	eam	8.5.6
compareit	in4	0.0.2
vscode-line-endings	jha	0.1.0
nbpreviewer	jit	1.0.0
binary-viewer	mom	0.0.1
python	ms-	2018.9.1
csharp	ms-	1.16.2
PowerShell	ms-	1.9.0
autodocstring	njp	0.2.3
partial-diff	ryu	1.4.0
git-merger	sha	0.4.1
vscodeintellicode	Vis	1.0.5
markdown-all-in-one	yzh	1.6.2
t4-support	zbe	0.4.2

[brettcannon]

@joseortiz3 we don’t support multi-process debugging yet (it’s coming).

[rtanase]

it might be coming from matplotlib, but I’m not sure. The funny thing is that it’s not always reproducible, for the same script it was working fine initially, then subsequent debugging sessions on the same script was giving me this error. After it started throwing the error, than any other debug retries would have the same result until I kill the Microsoft.Python.LanguageServer.exe process.

Also, in the original bug that I created on the general vscode repo, another user commented that they’re seeing same issue.