How to Avoid Errors in KNIME Expression Node When a Required Column is Missing After Pivot?

I have a question about KNIME. In the Expression node, my formula always uses the column “a”. However, the previous step is the result of a pivot operation, and the column “a” might not appear. If it doesn’t appear, the Expression node will throw an error. How can I avoid this error?

如何避免公式引用出错.knwf