Guard against undefined $container element in initMobile.js

In order to prevent spikes in JS errors, ensure that the $container
element exists before initializing the mobile media viewer.

Bug: T360781
Change-Id: I48a57e8008e7bd90e9fcfc783f3720f6f75520a7

resources/skins.minerva.scripts/initMobile.js

@@ -444,7 +444,12 @@ module.exports = function () {
 		// setup toc icons
 		mw.hook( 'wikipage.content' ).add( function ( $container ) {
 			// If the MMV module is missing or disabled from the page, initialise our version
-			if ( desktopMMV === null || desktopMMV === 'registered' ) {
+			if (
+				desktopMMV === null ||
+				desktopMMV === 'registered' &&
+				// T360781
+				$container &&
+				$container[ 0 ] ) {
 				initMediaViewer( $container[ 0 ] );
 			}