Non-persistent error banner when generating invitation with existing user's email address

Using Keybase 3.2.2-20190411230550+5262f90fd9 on Windows.

Trying to generate an invitation using a recipient email address causes an error banner to be displayed very briefly before disappearing and leaving no sign of the problem that the email address is already in use on Keybase.

Sets to reproduce:
In Keybase client, click Settings, Invitations.
Type in the email address of an existing Keybase user.
Click "Generate Invitation".
Notice that the red error banner flashes up too briefly to be read or understood.

Start in the Keybase client Settings > Invitations flow and reproduce the error with an existing user's email address on Windows. The work is done when the error banner remains visible long enough to read and clearly explains that the email address is already in use.

Written by the indexing model from the issue text.