TBR: reader/android: add null check to prevent app crash

Change-Id: I24a36bd2508947159565e267ba6e11382bccddf9
diff --git a/android/app/src/main/java/io/v/android/apps/reader/PdfViewerActivity.java b/android/app/src/main/java/io/v/android/apps/reader/PdfViewerActivity.java
index b39f77a..f94c2a6 100644
--- a/android/app/src/main/java/io/v/android/apps/reader/PdfViewerActivity.java
+++ b/android/app/src/main/java/io/v/android/apps/reader/PdfViewerActivity.java
@@ -125,7 +125,9 @@
 
                 DeviceMeta dm = getDeviceMeta();
                 mPdfView.setPage(dm.getPage());
-                mMenuItemLinkPage.setChecked(dm.getLinked());
+                if (mMenuItemLinkPage != null) {
+                    mMenuItemLinkPage.setChecked(dm.getLinked());
+                }
             }
 
             @Override