Check tabmodelselector null on reader mode callback

chrome/android/java/src/org/chromium/chrome/browser/dom_distiller/ReaderModeManager.java

Index: chrome/android/java/src/org/chromium/chrome/browser/dom_distiller/ReaderModeManager.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/dom_distiller/ReaderModeManager.java b/chrome/android/java/src/org/chromium/chrome/browser/dom_distiller/ReaderModeManager.java
index 27c8dfeef6f37791fed94700eb84725e94b7d701..48b178066a909adfac9583c6a77fae9f7a180dc7 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/dom_distiller/ReaderModeManager.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/dom_distiller/ReaderModeManager.java
@@ -439,6 +439,8 @@ public class ReaderModeManager extends TabModelSelectorTabObserver
      * @param reason The reason the panel is requesting to be shown.
      */
     protected void requestReaderPanelShow(StateChangeReason reason) {
+        if (mTabModelSelector == null) return;
+
         int currentTabId = mTabModelSelector.getCurrentTabId();
         if (currentTabId == Tab.INVALID_TAB_ID) return;
 
@@ -508,6 +510,8 @@ public class ReaderModeManager extends TabModelSelectorTabObserver
                 new DistillablePageUtils.PageDistillableDelegate() {
                     @Override
                     public void onIsPageDistillableResult(boolean isDistillable, boolean isLast) {
+                        if (mTabModelSelector == null) return;
+
                         ReaderModeTabInfo tabInfo = mTabStatusMap.get(readerTabId);
                         Tab readerTab = mTabModelSelector.getTabById(readerTabId);