fix #8302, disabling save button when can_add: false, can_overwrite: false (#9456)

superset-frontend/src/explore/components/QueryAndSaveBtns.jsx

@@ -24,7 +24,7 @@ import classnames from 'classnames';
 import Button from '../../components/Button';
 
 const propTypes = {
-  canAdd: PropTypes.string.isRequired,
+  canAdd: PropTypes.bool.isRequired,
   onQuery: PropTypes.func.isRequired,
   onSave: PropTypes.func,
   onStop: PropTypes.func,
@@ -49,7 +49,7 @@ export default function QueryAndSaveBtns({
   errorMessage,
 }) {
   const saveClasses = classnames({
-    'disabled disabledButton': canAdd !== 'True',
+    'disabled disabledButton': !canAdd,
   });
 
   let qryButtonStyle = 'default';