I can reproduce the problem consistently after killing and restarting Textra.

If this information helps you solve this issue, would you consider implementing my suggestion from months ago? It is: to provide a quick way to erase an unsent draft text from the main screen without having to: click the contact, Select All, hit backspace, and then return to the list of texts.

To reliably get a white menu on my phone:

0) make sure you have at least two custom Vibration Patterns available 

1) make sure Textra is not running

2) open Textra

3) bring up the settings menu

4) select Customize Notifications

5) select Vibration Patterns

6) bring up the Delete / Rename menu button next to either of the custom patterns. Do not choose either option

7) back out of Vibration Patterns 

8) repeat steps 5 thru 7

9) REPEAT STEPS 5 thru 6

On my phone this sequence results in a white menu every time. Note that you must repeat steps 5 and 6 THREE TIMES. 

Alternatively, you can do steps 1 thru 6, then back out to the main screen, then choose any contact, and modify that contact's Vibration Patterns - doing it this way does not require a third invocation of the Vibration Pattern menu.

I haven't experimented enough to know if these are the only ways to cause this, or if multiple patterns are required, but this is how my phone is set up. 

If you want more info on my phone, just ask. 

LG 30, Oreo, latest updates installed