fix: handle state change with missing currentWidgetId

There is potentially a race condition between prefetch triggering
onComplete and the onComplete for updated query from the state change.
This functionality could use a refactor/rethink.

Author: cbarber

app/javascript/components/widget-controller.jsx

@@ -113,16 +113,27 @@ export class WidgetController extends React.Component {
   };
 
   nextWidget = () => {
-    this.setState(({ nextWidgetId }) =>
-      nextWidgetId
-        ? {
-            currentWidgetId: nextWidgetId,
-            nextWidgetId: null,
-            prevWidgetId: null,
-            transitionTime: null,
-          }
-        : {},
-    );
+    this.setState(state => {
+      const { nextWidgetId } = state;
+      if (!nextWidgetId) {
+        // eslint-disable-next-line
+        console.error(
+          `FATAL: nextWidgetId is undefined for nextWidget call. state: ${state}`,
+        );
+        return {
+          currentWidgetId: null,
+          nextWidgetId: null,
+          prevWidgetId: null,
+          transitionTime: null,
+        };
+      }
+      return {
+        currentWidgetId: nextWidgetId,
+        nextWidgetId: null,
+        prevWidgetId: null,
+        transitionTime: null,
+      };
+    });
   };
 
   prevWidget = () => {