After summarizeWindows flint_df can't able to convert into spark df

I can able to convert spark df to flint df and apply the summarizeWindows function also working.
After applying any flint function , I can't able to access the df (like i can't convert back to spark df, or can't able to save the result file.

Even flint.show() or spark _df.show() getting error. But function is working correctly, i cant able to access the result df.