Fix wxGUI Vector Network Analysis Tool close dialog

[tmszi]

Reproduce:

1. On the Layer Manager menu go to -> Vector -> Network analysis -> Vector network analysis tool (open dialog)
2. Close dialog via click on the Quit toolbar icon tool

Error message appear in the Console page:

wx._core
.
wxAssertionError
:
C++ assertion "GetEventHandler() == this" failed at
../src/common/wincmn.cpp(478) 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 "/usr/lib/python3/dist-
packages/wx/lib/agw/customtreectrl.py", line 7997, in
OnInternalIdle

if not self.HasAGWFlag(TR_MULTIPLE) and not
self.GetSelection():
SystemError
:
PyEval_EvalFrameEx returned a result with an error set

Same as #411.

[petrasovaa]

Merged to 78.