Problem with ShowModelessDialog and textbox multiline (Enter don't work)

Hi,
i have a mysterious probleme with the ShowModelessDialog Funktion and a multiline textbox which the Enter key don't work.
When I change the call with ShowDialog, than work it with the Enter key and the multiline textbox but when i open and close my dialog 2 times than i couldn't open any Dialog more and this do with the ShowModelessDialog.
I have a circle of problem and i don't know how i can solve this.
Thanks for your help

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