Unity Pro V13.0, Possible null value reported on DFB output.

This is true from Unity Pro V13.0 and will be corrected in Unity Pro V14.0, an HF has been created and published for Unity Pro V13.1, V13.1_HF0470178R_ DFB output pins

Each and every condition must be met to have such issue and so occurrence is very low.
This happens if:
The Enable implicit type conversion is checked (Tools/Project Settings/Program/Languages/Common)
An Output with a link is present above the Output involved
Source data type of this link and Variable data type of the DFB output are different.

Unchecking the option Enable implicit type conversion will solve the issue.
About Enable implicit type conversion option, thanks to take into account the following NOTICE extracted from Unity Pro documentation:

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