When a request type tied to a form is deleted, it's not possible to perform changes to the form

Issue Summary

After deleting a request type tied to a form, you will not be able to perform any changes on the form because during the saving process, the request type isn't found. 

Steps to Reproduce

1. Add a request type to the form.
2. Delete the request type.
3. Try to save the form.

Expected Results

The request type should be removed from the form after the deletion or a message should appear informing this.

Actual Results

A generic message will appear on the UI:

We couldn't save your changes.

Select *Save changes* to try again 

But you can see the error below on the browser dev tools:

{
    "errors": [
        {
            "errorMessage": "The request type you are trying to view does not exist."
        }
    ],
    "reasonKey": "sd.customerview.error.requestTypeNotFound",
    "reasonCode": "404"
}  

Workaround

Click on the Clear selection within the request type list in the form settings.