Resolve error closing bluprint full-page modals

The Apps tab on the Course settings page uses `page` to manipulate its
contents. Unfortunatly this interferred with operation on the other
tabs.

fixes ADMIN-1000

test plan:
  - follow the steps in the ticket.
  > expect the UI to work as expected
  > expect no js error in the devtools console

Change-Id: Ic046d926c6fa040d081e920efd0b2c0f4c557c9d
Reviewed-on: https://gerrit.instructure.com/151025
Tested-by: Jenkins
Reviewed-by: Carl Kibler <ckibler@instructure.com>
Reviewed-by: Dan Minkevitch <dan@instructure.com>
QA-Review: Deepeeca Soundarrajan <dsoundarrajan@instructure.com>
Product-Review: Ed Schiebel <eschiebel@instructure.com>

app/jsx/external_apps/router.js

@@ -63,13 +63,17 @@ import regularizePathname from '../external_apps/lib/regularizePathname'
   };
 
   const renderConfigurations = (ctx) => {
-    ReactDOM.render(
-      <Root>
-        <Configurations
-          pathname={ctx.pathname}
-          env={window.ENV} />
-      </Root>
-    , targetNodeToRenderIn);
+    // router.start is only called when loading the Apps tab
+    // so we don't want to try anything here that hasn't happened.
+    if (targetNodeToRenderIn) {
+      ReactDOM.render(
+        <Root>
+          <Configurations
+            pathname={ctx.pathname}
+            env={window.ENV} />
+        </Root>
+      , targetNodeToRenderIn);
+    }
   }
 
   /**