Fix NPE during activity recreation

Bug: 13630676
Change-Id: I3b68d5f21a4b77eab5b5493dcde670ad08cdc372
diff --git a/src/com/android/dialer/DialtactsActivity.java b/src/com/android/dialer/DialtactsActivity.java
index 363adda..32a22c9 100644
--- a/src/com/android/dialer/DialtactsActivity.java
+++ b/src/com/android/dialer/DialtactsActivity.java
@@ -970,7 +970,9 @@
 
     @Override
     public void setDialButtonEnabled(boolean enabled) {
-        mDialButton.setEnabled(enabled);
+        if (mDialButton != null) {
+            mDialButton.setEnabled(enabled);
+        }
     }
 
     @Override