Add handling for missing value

Author: SimonErm

routers/web/auth/auth.go

@@ -387,8 +387,12 @@ func getUserName(gothUser *goth.User) (string, error) {
 	case setting.OAuth2UsernameEmail:
 		return user_model.NormalizeUserName(strings.Split(gothUser.Email, "@")[0])
 	case setting.OAuth2UsernamePreferredUsername:
-		preferredUsername := gothUser.RawData["preferred_username"].(string)
-		return user_model.NormalizeUserName(preferredUsername)
+		preferredUsername, exists := gothUser.RawData["preferred_username"]
+		if exists {
+			return user_model.NormalizeUserName(preferredUsername.(string))
+		} else {
+			return "", fmt.Errorf("preferred_username is missing in received user data but configured as username source. Check if OPENID_CONNECT_SCOPES contains profile")
+		}
 	case setting.OAuth2UsernameNickname:
 		return user_model.NormalizeUserName(gothUser.NickName)
 	default: // OAuth2UsernameUserid