execute failed ( nullpointerexception ) null knime in node string to doc

hello everyone
i used a loop to read 117 text file as a table row and wanted to use string to document node but i receive this error.
execute failed ( nullpointerexception ) null
the title is not a big text but the text is large. what should i do to solve the problem.
thanks for helping.

Hi @adelehasadi1

can you post the full stack trace here? Than I can have a engineer check this.

Which KNIME Version are your currently using?