add css to the data object to be saved (#2188)

superset/assets/javascripts/dashboard/components/Controls.jsx

@@ -18,7 +18,7 @@ class Controls extends React.PureComponent {
   constructor(props) {
     super(props);
     this.state = {
-      css: props.dashboard.css,
+      css: props.dashboard.css || '',
       cssTemplates: [],
     };
   }

superset/assets/javascripts/dashboard/components/SaveModal.jsx

@@ -40,6 +40,7 @@ class SaveModal extends React.PureComponent {
   saveDashboardRequest(data, url, saveType) {
     const dashboard = this.props.dashboard;
     const saveModal = this.modal;
+    Object.assign(data, { css: this.props.css });
     $.ajax({
       type: 'POST',
       url,