fix: remove sticky tooltip when query returns no data in explore view (…

…#42)

Tap into the newly added `willUnmount` hook to clean up sticky tooltip that nvd3 adds to `body` directly. This removes such tooltips that were left behind in the explore view with the new query returns 'no data'.

superset-frontend/temporary_superset_ui/superset-ui/plugins/superset-ui-plugins/packages/superset-ui-legacy-preset-chart-nvd3/src/ReactNVD3.js

@@ -18,5 +18,10 @@
  */
 import { reactify } from '@superset-ui/chart';
 import Component from './NVD3Vis';
+import { hideTooltips } from './utils';
 
-export default reactify(Component);
+function willUnmount() {
+  hideTooltips(true);
+}
+
+export default reactify(Component, { willUnmount });