Crash when removing tests from testing framework

[liquidmetal9015]

Description

When removing tests from the active test list I've run into a number of crashes, the most reproducible being the following one.

Add 3 or 4 tests to the list and then repeatedly press the remove button, usually some kind of crash will occur on the second or third remove such as the following.

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/clindbeck9/robocup/robocup-software/soccer/gameplay/test_system.py", line 47, in removeTest
    elif index == self._testIndex and self._testNode.status == test_list.Status.running:
AttributeError: 'NoneType' object has no attribute 'status'
terminate called after throwing an instance of 'std::runtime_error*'
makefile:65: recipe for target 'backend-headless-simulator-soccer' failed
make: *** [backend-headless-simulator-soccer] Aborted (core dumped)