Fix taking snapshot of local iframes

  contentDocument of some local iframes can be null,
  in this case, we should ignore these iframes.

Author: mika

src/js/snapshot/snapshot.js

@@ -364,14 +364,24 @@ async function takeSnapshotOfCurrNode_HTML(node, params) {
             // There still has a posibility that it's content
             // was generated by JS rather than just blank.
 
-            const children = [node.contentDocument];
-            const newDomParams = Object.assign(
-              {}, domParams,
-              (params.domParams_localFrame || {}),
-              {frameInfo: newFrameInfo}
-            );
-            const childParams = Object.assign({}, params, {domParams: newDomParams});
-            return {snapshot, children, childParams};
+            if (node.contentDocument) {
+              const children = [node.contentDocument];
+              const newDomParams = Object.assign(
+                {}, domParams,
+                (params.domParams_localFrame || {}),
+                {frameInfo: newFrameInfo}
+              );
+              const childParams = Object.assign({}, params, {domParams: newDomParams});
+              return {snapshot, children, childParams};
+            } else {
+              // Some local frames are used to run scripts only (something called iframe jax),
+              // these scripts may cause the contentDocument become null,
+              // for instance: (changing the URL of the document)
+              snapshot.childNodes = [];
+              snapshot.errorMessage = 'can not access contentDocument of iframe';
+              snapshot.render = 'ignore';
+              return {snapshot};
+            }
           }