Combining Two Columns into One

Hey guys,

I've got two seperate columns containing "hour registration codes".
These two columns should be transformed into one.
If Column A (Ontvanger) is Null, the code is available in Column B (Aan/afw).

If Column B is Null, the code is available in Column A.

I thought I could fix this concatenating the two columns in Power Query.

The result was exactly the same as Column A though.
Any ideas on how to fix this? 

With kind regards,

Lazzanova