Fix a NPE when clicking encryption interstitial header

Pref tree handler cannot assume preference has non-null key.

Change-Id: I790e75f3b4c888e2a48b297bea89cda0b8dbaabd
Fix: 35855587
Test: runtest --path tests/app/src/com/android/settings/EncryptionInterstitialTest.java

tests/app/src/com/android/settings/EncryptionInterstitialTest.java

@@ -0,0 +1,53 @@
+/*
+ * Copyright (C) 2017 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.settings;
+
+import android.app.Instrumentation;
+import android.content.Context;
+import android.content.Intent;
+import android.support.test.InstrumentationRegistry;
+import android.support.test.filters.SmallTest;
+import android.support.test.runner.AndroidJUnit4;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import static android.support.test.espresso.Espresso.onView;
+import static android.support.test.espresso.matcher.ViewMatchers.withId;
+
+@RunWith(AndroidJUnit4.class)
+@SmallTest
+public class EncryptionInterstitialTest {
+
+    private Instrumentation mInstrumentation;
+    private Context mContext;
+
+    @Before
+    public void setUp() {
+        mInstrumentation = InstrumentationRegistry.getInstrumentation();
+        mContext = mInstrumentation.getTargetContext();
+    }
+
+    @Test
+    public void clickHeader_shouldNotCrash() {
+        mInstrumentation.startActivitySync(
+                new Intent().setClassName(mContext, EncryptionInterstitial.class.getName()));
+        onView(withId(R.id.encryption_interstitial_header));
+        // Nothing should happen
+    }
+}