migration.v17: add nil error check

models/migrations/v17.go

@@ -12,7 +12,7 @@ import (
 
 func removeInvalidProtectBranchWhitelist(x *xorm.Engine) error {
 	_, err := x.Exec("DELETE FROM protect_branch_whitelist WHERE protect_branch_id = 0")
-	if strings.Contains(err.Error(), "no such table") {
+	if err != nil && strings.Contains(err.Error(), "no such table") {
 		return nil
 	}
 	return err