fix issue with loading Yubikey aux file. closes #2880

src/keepass2android-app/PasswordActivity.cs

@@ -421,6 +421,12 @@ namespace keepass2android
 							{
 								var iocAux = GetDefaultAuxLocation();
                                 LoadFile(iocAux);
+
+                                if (Activity._chalInfo == null)
+                                {
+                                    throw new Java.Lang.Exception("Failed to load challenge aux file");
+                                }
+
                             }
 							catch (Exception e)
 							{