Fix NPE in KeyDetector

This is actually a follow up of I9290974821.

Bug: 13215075
Change-Id: Ib7eef55fd1cfbb05d06aeeb0726bc10c87b07d85

java/src/com/android/inputmethod/keyboard/KeyDetector.java

@@ -86,6 +86,9 @@ public class KeyDetector {
      * @return the key that the touch point hits.
      */
     public Key detectHitKey(final int x, final int y) {
+        if (mKeyboard == null) {
+            return null;
+        }
         final int touchX = getTouchX(x);
         final int touchY = getTouchY(y);