Check for no PGP sign key selected

app/src/main/java/eu/faircode/email/FragmentCompose.java

@@ -1927,6 +1927,8 @@ public class FragmentCompose extends FragmentBase {
                                 }
                             } else if (OpenPgpApi.ACTION_GET_SIGN_KEY_ID.equals(data.getAction())) {
                                 pgpSignKeyId = result.getLongExtra(OpenPgpApi.EXTRA_SIGN_KEY_ID, -1);
+                                if (pgpSignKeyId == 0)
+                                    throw new IllegalArgumentException(context.getString(R.string.title_no_sign_key));
                                 db.identity().setIdentitySignKey(identity.id, pgpSignKeyId);
 
                                 // Get public key

app/src/main/res/values/strings.xml

@@ -814,6 +814,7 @@
     <string name="title_user_interaction">Transferring to OpenKeychain</string>
     <string name="title_signed_data">Verify the signature to show the message text</string>
     <string name="title_not_encrypted">Message is not signed or encrypted</string>
+    <string name="title_no_sign_key">No sign key selected</string>
     <string name="title_reset_sign_key">Reset sign key</string>
 
     <string name="title_signature_none">Message not signed</string>