Issue: 🐛Bug report | Crash if interacting with Build > Mesh after closing other model

[stevenlabelle]

Description: FBS will crash if the Build > Mesh panel is interacted with if there is no mesh in the current tab but there was one in a prior tab.
Version: Dev version 2.7.0.1baaa20fa

To reproduce:

1. Open a new model and create a primitive (e.g., cube).
2. Click the Mesh tab under Build > Mesh.
3. Close the model tab without saving.
4. Open a new model.
5. The Build > Mesh tab will be open this time.
6. Click on a field in the Build > Mesh tab (e.g., "Nx") then click anywhere else on screen. The model will crash.