Fix NFC crasher from accessing a destroyed activity.

device/nfc/android/java/src/org/chromium/device/nfc/NfcImpl.java

Index: device/nfc/android/java/src/org/chromium/device/nfc/NfcImpl.java
diff --git a/device/nfc/android/java/src/org/chromium/device/nfc/NfcImpl.java b/device/nfc/android/java/src/org/chromium/device/nfc/NfcImpl.java
index 35a73205968cabea4f543f80833fec2eaa5fdd42..fd6fbdc320430247ecce6abb661a1cc72b2f78a5 100644
--- a/device/nfc/android/java/src/org/chromium/device/nfc/NfcImpl.java
+++ b/device/nfc/android/java/src/org/chromium/device/nfc/NfcImpl.java
@@ -362,11 +362,12 @@ public class NfcImpl implements Nfc {
      * Disables reader mode.
      * @see android.nfc.NfcAdapter#disableReaderMode
      */
+    @TargetApi(Build.VERSION_CODES.KITKAT)
     private void disableReaderMode() {
         if (Build.VERSION.SDK_INT < Build.VERSION_CODES.KITKAT) return;
 
         mReaderCallbackHandler = null;
-        if (mActivity != null) {
+        if (mActivity != null && !mActivity.isDestroyed()) {
             mNfcAdapter.disableReaderMode(mActivity);
         }
     }