[Form][BUG]Button missing getErrorsAsString() fixes #8084

Debug: Not calling undefined method anymore.
If the form contained a submit button the call would fail and the debug of the form wasn't possible.
Now it will work in all cases.
This fixes #8084 

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8084
| License       | MIT