Empty rectangle crashes when assigning

[hobbitsyfeet]

To reproduce issue:

1. add tab
2. draw square
3. click on information window
4. click on video window
5. space bar
   ERROR:
   Exception has occurred: error
   OpenCV(4.1.1) C:\projects\opencv-python\opencv\modules\core\src\dxt.cpp:3335: error: (-215:Assertion failed) type == CV_32FC1 || type == CV_32FC2 || type == CV_64FC1 || type == CV_64FC2 in function 'cv::dft'
   File "C:\Users\legom\Documents\GitHub\PeopleTracker\src\multitracker.py", line 170, in assign
   self.tracker.init(frame, self.init_bounding_box)
   File "C:\Users\legom\Documents\GitHub\PeopleTracker\src\multitracker.py", line 822, in
   tracker_list[selected_tracker].assign(frame, trackerName)

Suspected behaviour:
When this is happening, it reads that there is no rectangle created to track, so it tries to create a tracker on a (0x0) rectangle, and crashes.

[hobbitsyfeet]

The bounding box failing to be set re-enables the UI and prompts to try again.

Further investigation needs to happen to see if this fixes a related issue where tabs remain disabled.