Issue #3416141 by gorkagr, Jaypan, Wim Leers: big_pipe.js' checkMutation()...

Issue #3416141 by gorkagr, Jaypan, Wim Leers: big_pipe.js' checkMutation() does not check if node exists before using it

core/modules/big_pipe/js/big_pipe.js

@@ -89,7 +89,8 @@
    */
   function checkMutation(node) {
     return Boolean(
-      node.nodeType === Node.ELEMENT_NODE &&
+      node &&
+        node.nodeType === Node.ELEMENT_NODE &&
         node.nodeName === 'SCRIPT' &&
         node.dataset &&
         node.dataset.bigPipeReplacementForPlaceholderWithId &&