Fix(SideContainerToggler.js): Check if the component exists on init

modules/UI/side_pannels/SideContainerToggler.js

     init(eventEmitter) {
         this.eventEmitter = eventEmitter;
 
-        // Adds a listener for the animation end event that would take care
-        // of hiding all internal containers when the extendedToolbarPanel is
+        // We may not have a side toolbar container, for example, in
+        // filmstrip-only mode.
+        const sideToolbarContainer
+            = document.getElementById("sideToolbarContainer");
+
+        if (!sideToolbarContainer)
+            return;
+
+        // Adds a listener for the animationend event that would take care of
+        // hiding all internal containers when the extendedToolbarPanel is
         // closed.
-        document.getElementById("sideToolbarContainer")
-            .addEventListener("animationend", function(e) {
-                if(e.animationName === "slideOutExt")
+        sideToolbarContainer.addEventListener(
+            "animationend",
+            function(e) {
+                if (e.animationName === "slideOutExt")
                     $("#sideToolbarContainer").children().each(function() {
                         if ($(this).hasClass("show"))
                             SideContainerToggler.hideInnerContainer($(this));
                     });
-            }, false);
+            },
+            false);
     },
 
     /**
     }
 };
 
-export default SideContainerToggler;
+export default SideContainerToggler;