Fix bug that crashed noVNC when noVNC canvas was null

novnc/app/camera-receiver.js

@@ -408,6 +408,7 @@ document.addEventListener('DOMContentLoaded', function() {
 
     function adjustVideoGeometry() {
         var canvas = document.querySelector('canvas');
+        if (canvas) {
             var canvasRect = canvas.getBoundingClientRect();
             var vncWidth = canvas.width;
             var vncHeight = canvas.height;
@@ -439,6 +440,9 @@ document.addEventListener('DOMContentLoaded', function() {
                     }
                 });
             }
+        } else {
+            console.error('adjustVideoGeometry canvas was null');
+        }
     }
 });