[DevTools] Add null check to target.parentTarget() in NetworkLog

I forgot to add a null check as Dmitry suggested here:
https://chromium-review.googlesource.com/c/chromium/src/+/1357666/4#message-bc921121ce2c839de5e6a614a51ae48952d1a53a

Bug: 908592
Change-Id: I32d46fd2feef20b7c129ea5bf7075fd386befcb9
Reviewed-on: https://chromium-review.googlesource.com/c/1471220
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Reviewed-by: Erik Luo <luoe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632354}

third_party/blink/renderer/devtools/front_end/sdk/NetworkLog.js

@@ -330,7 +330,8 @@ SDK.NetworkLog = class extends Common.Object {
       if (!networkManager)
         continue;
       const target = networkManager.target();
-      if (!(target.type() === SDK.Target.Type.Worker && target.parentTarget().type() === SDK.Target.Type.ServiceWorker))
+      if (!(target.type() === SDK.Target.Type.Worker && target.parentTarget() &&
+            target.parentTarget().type() === SDK.Target.Type.ServiceWorker))
         continue;
 
       // If there is a matching request that came before this one, keep it.