Object reference not set to an instance of an object

I encoutered same problem today.

In my case there was problem with data model.

Simply, my data sources are coming from various excel files which are stored in various Sharepoint Folders.

For some reason - Power BI (PQ) has pulled all columns available in excel file which is around 10000 (no matter those were blank). There was a step to add custom column "Connector", it could not be added as PQ reached limit of columns (there was no error notification in PQ which is strange).

Because of that, data from this table did not have any relationship with other tables and refreshing was impossible.

I fixed that by removing all null columns.

Then custom column could be added and relationship with other columns has been restored.