PythonVideoAnnotator crashes when removing the last video/object from the list after making a selection in the Tracking module.

[miguelgaspar24]

Bug description
PythonVideoAnnotator crashes when removing the last video/object from the list after making a selection in the Tracking module.

To Reproduce

1. Load a project or open a video.

2. Open the "Track objects" module.

3. Check the box and left-click any one of the loaded project's videos to select it, then close the "Tracking object" window (4).

4. Remove each video/object one by one.

Output error trace:

Traceback (most recent call last):
 File "c:\users\miguel gaspar\desktop\my_virtual_environments\video_annotator\lib\site-packages\pyforms_gui\controls\control_checkboxlist.py", line 100, in __itemSelectionChanged
 self.selection_changed_event()
 File "c:\users\miguel gaspar\desktop\my_virtual_environments\video_annotator\lib\site-packages\pythonvideoannotator_models_gui\dialogs\objects\objects.py", line 35, in __selection_changed_event
 def __selection_changed_event(self): self.video_selection_changed_event()
 File "c:\users\miguel gaspar\desktop\my_virtual_environments\video_annotator\lib\site-packages\pythonvideoannotator_models_gui\dialogs\datasets\datasets.py", line 47, in __video_selection_changed_event
 self.video_selection_changed_event()
 File "c:\users\miguel gaspar\desktop\my_virtual_environments\video_annotator\lib\site-packages\pythonvideoannotator_module_tracking\tracking_window.py", line 103, in __video_selection_changed_event
 self._filter.video = video
 File "c:\users\miguel gaspar\desktop\my_virtual_environments\video_annotator\lib\site-packages\pythonvideoannotator_module_tracking\module_mcvgui\dialogs\tracking_filter.py", line 135, in video
 self.video_capture = value.video_capture if value is not None else None
 File "c:\users\miguel gaspar\desktop\my_virtual_environments\video_annotator\lib\site-packages\mcvgui\dialogs\simple_filter\simple_filter.py", line 220, in video_capture
 self._player.value = value
 File "c:\users\miguel gaspar\desktop\my_virtual_environments\video_annotator\lib\site-packages\pyforms_gui\controls\control_player\control_player.py", line 449, in value
 self.refresh()
 File "c:\users\miguel gaspar\desktop\my_virtual_environments\video_annotator\lib\site-packages\pyforms_gui\controls\control_player\control_player.py", line 161, in refresh
 frame = self.process_frame_event(self._current_frame.copy())
 File "c:\users\miguel gaspar\desktop\my_virtual_environments\video_annotator\lib\site-packages\mcvgui\dialogs\simple_filter\simple_filter.py", line 142, in __process_frame
 frame_index = self._player.video_index-1
TypeError: unsupported operand type(s) for -: 'NoneType' and 'int'

Expected behavior
The expectation would be for the program to not crash.

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