Fix two exceptions in JavascriptAutoComplete.js

1. "Cannot read property 'length' of null" at line 95
Event though the args has a length of 1, that item could be null and causing a[0].length to throw an error.

2. "Cannot read property 'className' of null" at line 148
The receiverObj has a type of RemoteObject | null. We do an early return in the case of null.


Change-Id: I6b43248fd16c0867f9d4b1252621f5d4c8bbb551
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2444012
Reviewed-by: Simon Zünd <szuend@chromium.org>
Commit-Queue: Christy Chen <chrche@microsoft.com>

front_end/object_ui/JavaScriptAutocomplete.js

@@ -92,7 +92,7 @@ export class JavaScriptAutocomplete {
       return (result && !result.exceptionDetails && result.object) ? result.object : null;
     }, functionCall.functionName);
     executionContext.runtimeModel.releaseObjectGroup('argumentsHint');
-    if (!args.length || (args.length === 1 && !args[0].length)) {
+    if (!args.length || (args.length === 1 && (!args[0] || !args[0].length))) {
       return null;
     }
     return {args, argumentIndex: functionCall.argumentIndex};
@@ -145,6 +145,9 @@ export class JavaScriptAutocomplete {
       return uniqueSignatures;
     }
     const receiverObj = await receiverObjGetter();
+    if (!receiverObj) {
+      return [];
+    }
     const className = receiverObj.className;
     if (javaScriptMetadata.signaturesForInstanceMethod(name, className)) {
       return javaScriptMetadata.signaturesForInstanceMethod(name, className);