Check if asyncparams is null for document mode UI initialization

The if statement checks for the tabtobereparented to be null
without checking the params itself.

BUG=590637

Review URL: https://codereview.chromium.org/1746933002

Cr-Commit-Position: refs/heads/master@{#378249}

Author: yusufo

chrome/android/java/src/org/chromium/chrome/browser/document/DocumentActivity.java

@@ -578,7 +578,7 @@ private void initializeUI() {
         boolean isCreatedWithWebContents = asyncParams != null
                 && asyncParams.getWebContents() != null;
 
-        if (params.getTabToReparent() != null) {
+        if (params != null && params.getTabToReparent() != null) {
             mTab = params.getTabToReparent();
             mTab.reparentToActivity(this, new DocumentTabDelegateFactory());
         } else {