Closing of d.mon crashes

[neteler]

In G78/master simple closing of d.mon leads to this error messages:

GRASS 7.8.dev (nc_spm_08):~ > d.mon wx0
GRASS 7.8.dev (nc_spm_08):~ > wx._core.wxAssertionError: C++ assertion "GetEventHandler() == this" failed at ../src/common/wincmn.cpp(477) in ~wxWindowBase(): any pushed event handlers must have been removed

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/mneteler/software/grass78_git/dist.x86_64-pc-linux-gnu/gui/wxpython/mapdisp/main.py", line 565, in OnExit
    for f in six.itervalues(monFile):
  File "/usr/local/lib/python3.7/site-packages/six.py", line 584, in itervalues
    return iter(d.values(**kw))
SystemError: <built-in method values of dict object at 0x7f69f04d74b0> returned a result with an error set

[veroandreo]

This is a duplicate of #3897