[Custom Tab] Fix a crash when getting a screenshot of a tab

chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabObserver.java

Index: chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabObserver.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabObserver.java b/chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabObserver.java
index 44dd1d005759a643ca340c647fe683fa2217ab0b..b4ad78c25271db6d6aee85e8ee68abe89bb3c663 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabObserver.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabObserver.java
@@ -188,6 +188,7 @@ class CustomTabObserver extends EmptyTabObserver {
             @Override
             public void run() {
                 if (!tab.isHidden() && mCurrentState != STATE_RESET) return;
+                if (tab.getWebContents() == null) return;
                 tab.getWebContents().getContentBitmapAsync(
                         Bitmap.Config.ARGB_8888, mScaleForNavigationInfo, new Rect(), callback);
                 mScreenshotTakenForCurrentNavigation = true;