Hello Azbader,
As the traceback suggest this is due to your params not lining up exactly with the FDTD mesh. In topology optimization you need to have the opt_region(x,y), be the same size as the x and y points defined in python. For this reason it is critical to use a mesh override, or uniform mesh with topology.
Inverse Design of a Splitter Using Topology Optimization
Optimizable Geometry - Python API
This is a common issue in topology set-up, if you change the FDTD simulation and the python params to align exactly you should not see that error.
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