Panels: Fixed crashing dashboards with panel links (#18430)

* ReactPanels: Fixed panel header tooltip rendering crash

* Added unit test

* Improved test

packages/grafana-ui/src/components/index.ts

 export { DeleteButton } from './DeleteButton/DeleteButton';
 export { Tooltip } from './Tooltip/Tooltip';
-export { PopperController } from './Tooltip/PopperController';
+export { PopperController, PopperContent } from './Tooltip/PopperController';
 export { Popper } from './Tooltip/Popper';
 export { Portal } from './Portal/Portal';
 export { CustomScrollbar } from './CustomScrollbar/CustomScrollbar';

public/app/features/dashboard/dashgrid/PanelHeader/PanelHeaderCorner.test.tsx

+import React from 'react';
+import { shallow } from 'enzyme';
+import { PanelHeaderCorner } from './PanelHeaderCorner';
+import { PanelModel } from '../../state';
+
+describe('Render', () => {
+  it('should render component', () => {
+    const panel = new PanelModel({});
+    const links: any[] = [
+      {
+        url: 'asd',
+        title: 'asd',
+      },
+    ];
+
+    const wrapper = shallow(<PanelHeaderCorner panel={panel} links={links} />);
+    const instance = wrapper.instance() as PanelHeaderCorner;
+
+    expect(instance.getInfoContent()).toBeDefined();
+  });
+});

public/app/features/dashboard/dashgrid/PanelHeader/PanelHeaderCorner.tsx

 import React, { Component } from 'react';
 
 import { renderMarkdown } from '@grafana/data';
-import { Tooltip, ScopedVars } from '@grafana/ui';
+import { Tooltip, ScopedVars, PopperContent } from '@grafana/ui';
 import { DataLink } from '@grafana/data';
 
 import { PanelModel } from 'app/features/dashboard/state/PanelModel';
@@ -44,7 +44,7 @@ export class PanelHeaderCorner extends Component<Props> {
 
   getInfoContent = (): JSX.Element => {
     const { panel } = this.props;
-    const markdown = panel.description;
+    const markdown = panel.description || '';
     const linkSrv = new LinkSrv(templateSrv, this.timeSrv);
     const interpolatedMarkdown = templateSrv.replace(markdown, panel.scopedVars);
     const markedInterpolatedMarkdown = renderMarkdown(interpolatedMarkdown);
@@ -71,7 +71,7 @@ export class PanelHeaderCorner extends Component<Props> {
     );
   };
 
-  renderCornerType(infoMode: InfoMode, content: string | JSX.Element) {
+  renderCornerType(infoMode: InfoMode, content: PopperContent<any>) {
     const theme = infoMode === InfoMode.Error ? 'error' : 'info';
     return (
       <Tooltip content={content} placement="top-start" theme={theme}>
@@ -95,7 +95,7 @@ export class PanelHeaderCorner extends Component<Props> {
     }
 
     if (infoMode === InfoMode.Info || infoMode === InfoMode.Links) {
-      return this.renderCornerType(infoMode, this.getInfoContent());
+      return this.renderCornerType(infoMode, this.getInfoContent);
     }
 
     return null;