Fix NPE in TabContext#createCurrentContext

Without a deep dive investigation, I could not tell what is the root
cause of this NPE. This CL adds a null check and skips the null tab to
calm down the crash rate. A TODO is added to remind us to revisit this
NPE.

TBR=wychen@chromium.org

Bug: 1146320
Change-Id: Iaeb5de83bf3fe95c1adafcd973c503acbfcf93aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532867
Commit-Queue: Mei Liang <meiliang@chromium.org>
Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Reviewed-by: Yue Zhang <yuezhanggg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826491}

chrome/android/features/tab_ui/java/src/org/chromium/chrome/browser/tasks/tab_management/suggestions/TabContext.java

@@ -216,6 +216,12 @@ public class TabContext {
         // add it to a group it belongs to.
         for (int i = 0; i < tabModelFilter.getCount(); i++) {
             Tab currentTab = tabModelFilter.getTabAt(i);
+
+            assert currentTab != null : "currentTab should not be null";
+
+            // TODO(crbug.com/1146320): Investigate the NPE.
+            if (currentTab == null) continue;
+
             List<Tab> relatedTabs = tabModelFilter.getRelatedTabList(currentTab.getId());
 
             if (relatedTabs.size() > 1) {