STK: Check that main menu exists before retrieval

Bug: 19282177
Change-Id: Ie47020d49abef0c2296b96e5b3a78445e6e44fb6
diff --git a/src/com/android/stk/StkAppService.java b/src/com/android/stk/StkAppService.java
index 7410f81..1714d6d 100644
--- a/src/com/android/stk/StkAppService.java
+++ b/src/com/android/stk/StkAppService.java
@@ -432,7 +432,7 @@
      */
     Menu getMainMenu(int slotId) {
         CatLog.d(LOG_TAG, "StkAppService, getMainMenu, sim id: " + slotId);
-        if (slotId >=0 && slotId < mSimCount) {
+        if (slotId >=0 && slotId < mSimCount && (mStkContext[slotId].mMainCmd != null)) {
             return mStkContext[slotId].mMainCmd.getMenu();
         } else {
             return null;