Prompt text is erased upon backspace when the prompt text is a secret and its mask is null.

**Describe the bug**
Prompt text is erased upon backspace when the prompt text is a secret and its mask is null.

**To Reproduce**
Create secret Prompt with a null mask. Show the Prompt. Enter a few characters, then backspace. The prompt text will be deleted.

**Expected behavior**
Prompt text deletion should stop after the ultimate character of the prompt text.

