Exception when using default "Monitor" selection in 3.2 pre-release simulator

[gonzrl]

Describe the bug
When using the 3.2 pre-release simulator, an exception is thrown if a variable has "Monitor" selected by default and remains checked when running the simulator.

To Reproduce
Steps to reproduce the behavior:

1. Create a model with a variable that has both "Allow Monitor in Simulator" and "Monitor by Default" checked
2. Open the model in the 3.2 pre-release simulator
3. Run the simulator without changing the default selection