model_builder - run model runs forever

[komoseid]

Describe the bug
Running code snippet 237-244 in simulation_gaussian_data.Rmd opens a window, if you press "run" under "model" (no other options?) the window closes and a forever loading wheel comes up. If you cancel the running code the error
not-yet-implemented method for <dgCMatrix> %*% <NULL>
shows up.

Expected behavior
I expect some instructions of what is happening while loading, or a message if something is wrong.

To Reproduce
Running code snippet 237-244 in simulation_gaussian_data.Rmd and press "run" button

User Environment (please complete the following information):

• OS: macOS
• Version of R: 4.3.1
• Version of fdmr you are using: 0.1.1

[gareth-j]

Need to add some better checks to the model builder to ensure we don't have NULLs for the create_prediction_field function.