I am having that error when the process is calling PerformPreProcessing activity, located in @baseclass. In other hand, I have tried to bypass that problem replacing the table with a repeating layout, but error appears when I call a flow action over a row.
Tracing, I have discovered the next behaviour:
pyWorkPage is correct on the end of the activity pzPerformGridAction. Nevertheless, on the first step of the pzPrepareAssignment the page of the context (pyWorkPage.PageListPorperty(1) )is empty.