Why do I see a MATLAB Coder error when building with a Kalman filter?

When trying to build a MATLAB function that returns a struct with a Kalman filter, I get the following error:

Handle Classes in top-level outputs are not supported in MATLAB Coder. Output parameter 'output1' contains a handle class.

Answer 1

[MathWorks Support Team]

Kalman filters are a Handle Class Object, and currently can not be inputs or outputs to entry point functions for code generation.

https://www.mathworks.com/help/simulink/ug/how-working-with-matlab-classes-is-different-for-code-generation.html#btsyar3-1