[TYPO3-english] Form - preview in backend throws exception

[Thomas Boehm]

Hi all,

I'm trying to get the Form extension in version 8.7.9 to work. If I
create a simple form with only one text field I can put it on a page and
it shows in the frontend.

However, If I try to preview this form in the backend, Typo3 throws an
exception "#1297759968: Exception while property mapping at property
path "": No converter found which can be used to convert from "string"
to "array"." and in the Apache log I can see an error 500.

I also tried a blank Typo3 8.7.9 installation as well as the latest dev
version 8.7.10 from Github.