Fix crash that can occur when trying to display preview key

Fixes Crashlytics  [crash report](https://console.firebase.google.com/u/0/project/kmapro-ee779/crashlytics/app/android:com.tavultesoft.kmapro/issues/5b59a79f6007d59fcdced3c2)  that's caused by trying to display the preview key when the token is null.