Add additional prop validation to Tabs component (#8267)

The Tabs component expects the `children` prop to be either a single
Tab component or an array of Tab components, and the internal tab index
should always map onto one of these components. However, if an invalid
tab index was set, it was just returning the first tab contents.

Instead it will now validate that the tab being asked for does exist,
and throw an error otherwise.

ui/app/components/ui/tabs/tabs.component.js

@@ -42,6 +42,13 @@ export default class Tabs extends Component {
     const { children } = this.props
     const { activeTabIndex } = this.state
 
+    if (
+      (Array.isArray(children) && !children[activeTabIndex]) ||
+      (!Array.isArray(children) && activeTabIndex !== 0)
+    ) {
+      throw new Error(`Tab at index '${activeTabIndex}' does not exist`)
+    }
+
     return children[activeTabIndex]
       ? children[activeTabIndex].props.children
       : children.props.children