Prevent Forms Android app to restart

I have a forms app for android & iOS. When the app goes to the background (on Android) by using the back button and then tap the app icon, the app restarts.
My login 'window' shows and then the user cannot login, get a message "Unable to add window -- token XYZ is not valid; is your activity running?'
I have tried to change launch mode which i understood prevent app to restart but no luck yet.