PythonVideoAnnotator shows a blank canvas after closing and then reopening certain module windows.

[miguelgaspar24]

Bug description
PythonVideoAnnotator shows a blank canvas after closing and then reopening the "Track object", "Background calculation", "Mask edit" or "Path map" windows.

To Reproduce

1. Open one of the modules mentioned in the description, close the window, and then reopen the window.

"Track object"

"Background calculation"

"Mask edit"

"Path map"

Expected behavior
The expectation would be for the program continue to show the movie/masks/paths as it does on the first opening, instead of blanking, which makes accompanying modifications impossible.

Desktop and Versioning Details

OS: Microsoft Windows 10 Pro x64

Virtual Environment Python: 3.6.2

Python Video Annotator and Modules:

Python-video-annotator                                 3.306
Python-video-annotator-models                          0.8.82
Python-video-annotator-models-gui                      0.7.63
Python-video-annotator-module-background-finder        0.5.21
Python-video-annotator-module-contours-images          0.5.28
Python-video-annotator-module-create-paths             0.5.15
Python-video-annotator-module-deeplab                  0.902.21
Python-video-annotator-module-distances                0.5.18
Python-video-annotator-module-eventstats               0.5.15
Python-video-annotator-module-find-orientation         0.5.18
Python-video-annotator-module-import-export            0.5.23
Python-video-annotator-module-motion-counter           0.5.26
Python-video-annotator-module-path-editor              0.5.28
Python-video-annotator-module-path-map                 0.6.16
Python-video-annotator-module-regions-filter           0.5.18
Python-video-annotator-module-smooth-paths             0.5.19
Python-video-annotator-module-timeline                 0.6.26
Python-video-annotator-module-tracking                 0.6.38
Python-video-annotator-module-virtual-object-generator 0.6.26