How can I clear data set by PXLongOperation.SetCustomInfo("some text") after PXLongOperation.WaitCompletion(?)

I have an Action on a form that fetches some info from an external API inside a PXLongOperation.  I am putting the resulting json into custominfo with:

PXLongOperation.SetCustomInfo(json);

and retrieving after PXLongOperation with:

PXLongOperation.WaitCompletion(this.UID); 

if (PXLongOperation.GetCustomInfo(this.UID) is string json)
{
    f.JsonResult = json;
    Filter.Update(f);
}

The problem is that I have date fields in the filter section of the form that trigger the operation completed successfully message every time I change them after the initial action that calls the API completes.

I have tried putting the following immediately after setting f.JsonResult in the if statement:

PXLongOperation.SetCustomInfo(null);

and

PXLongOperation.SetCustomInfo(this.UID, null);

The first doesn’t seem to affect the data which I presumed is because it needs to know the UID and the second throws an error about ambiguity so I tried:

PXLongOperation.SetCustomInfo(this.UID, (string)null);

which clears the error but doesn’t clear the data.

If I debug and check PXLongOperation.GetCustomInfo() in a watch immediately after that line completes it still seems to be retaining the data.

Am I missing something simple here?

Thanks for any advice anyone has,

Phil