fix(ui/dygraph): add guard for dygraph (#12828)

ui/src/shared/components/dygraph/Dygraph.tsx

@@ -165,7 +165,7 @@ class Dygraph extends Component<Props, State> {
         {legendData && (
           <Legend {...legendData} seriesDescriptions={seriesDescriptions} />
         )}
-        {!!hoverTime && (
+        {!!hoverTime && !!this.dygraph && (
           <HoverTimeMarker x={this.dygraph.toDomXCoord(hoverTime)} />
         )}
         {this.nestedGraph}