Approach area shading bug

[nrankin18]

I was able to reproduce #30 again:

Steps to reproduce:

1. Open SimAware - approach area is displayed
2. Minimize SimAware to task bar for extended period of time (the screenshot above is from multiple hours, but I'm sure it could be shorter)
3. Reopen SimAware. The data displayed is "frozen" from the time the window was minimized.
4. Hover over approach so area is shaded and controller info is displayed.
5. When "frozen" data is updated to the current data (with the approach now closed), the boundary and controller name disappear, but shading remains until a refresh.