Load Project While Running Old Bug

[ElliotRoe]

Summary
If a user loads a new project while old threads are still running with a while true loop, the threads containing a while true loop still run even after the new project has been loaded in.
Expected Behavior
There should be no threads of the old project left running after the new project has been loaded
Steps to reproduce

1. New Project
2. Add the following code

penDown()
while True:
  goTo("_random_")
  wait(1)

3. Run the code
4. Press "File" > "Load from your computer" and select a pre saved project file
5. While there are no sprites that are from the old project, their will be pen strokes appearing in the backdrop