[Paint Preview] Fix crash in animation logic

There's a NullPointerException crash caused when a tab is closed while
paint preview is fading out. This fixes that.

Bug: 1133170
Change-Id: I3d3b025fdc70ea83113c09a054f7120555e51f98
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2437739
Commit-Queue: Mehran Mahmoudi <mahmoudi@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Auto-Submit: Mehran Mahmoudi <mahmoudi@chromium.org>
Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811750}

chrome/browser/paint_preview/android/java/src/org/chromium/chrome/browser/paint_preview/TabbedPaintPreviewPlayer.java

@@ -243,10 +243,14 @@ public class TabbedPaintPreviewPlayer implements TabViewProvider, UserData {
                 .setListener(new AnimatorListenerAdapter() {
                     @Override
                     public void onAnimationEnd(Animator animation) {
-                        mTab.getTabViewManager().removeTabViewProvider(
-                                TabbedPaintPreviewPlayer.this);
-                        mPlayerManager.destroy();
-                        mPlayerManager = null;
+                        if (mTab != null) {
+                            mTab.getTabViewManager().removeTabViewProvider(
+                                    TabbedPaintPreviewPlayer.this);
+                        }
+                        if (mPlayerManager != null) {
+                            mPlayerManager.destroy();
+                            mPlayerManager = null;
+                        }
                         mFadingOut = false;
                     }
                 });