Ugh. Sorry, I eventually did find a solution, but I think it was more of a kitchen-sink fix where I kept throwing everything at it until it stopped happening.

Pretty sure I deleted directly in SQL.

I just remembered the problem!

https://github.com/umbraco/Umbraco.Forms.Issues/issues/1140

If you're comfortable editing JSON and updating via SQL, you have to find the row in UFForms. Copy the Definition field into a text editor.

Then look for a section with "preValues". Mine looked like this:

"preValues": [
    {
        "value": "1"
    },
    {
        "value": "2"
    },
    {
        "value": "3"
    },
    {
        "value": "4"
    }
],

The problem is that there is no "caption" for the prevalues. So update it like this:

"preValues": [
    {
        "value": "1",
        "caption": "1"
    },
    {
        "value": "2",
        "caption": "2"
    },
    {
        "value": "3",
        "caption": "3"
    },
    {
        "value": "4",
        "caption": "4"
    }
],

Go back and update it in SQL, and the null reference error should be fixed.