I have done a few experiments.

I only get the error when the code is run at design time. At run time all is OK.

Of course a builder in a visual language should run at design time.

It does not seem to matter if the grid record source is set or not before I try to add columns.

I tryed adding a completely new grid. I gathered all needed information on the exisitng grid. I used it to add a completely new grid.

It failed when I tried to add the coulmns with the same error message as it did when I was trying to replace the exisitng grid and columns.