Fix NPE when removing PIN locked SIM during E911 call

There is the possibility of mCallback being null while
the Keyguard still has a PhoneStateListener registered
for onSimStateChanged callback. This can cause a NPE.

Bug: 66986066
Test: Manual, PIN/PUK locked SIM
Change-Id: I799c9521f787e800e259c3bdce9edee3f844f5f3
diff --git a/packages/SystemUI/src/com/android/keyguard/KeyguardSimPukView.java b/packages/SystemUI/src/com/android/keyguard/KeyguardSimPukView.java
index 826a03d..fddb12d 100644
--- a/packages/SystemUI/src/com/android/keyguard/KeyguardSimPukView.java
+++ b/packages/SystemUI/src/com/android/keyguard/KeyguardSimPukView.java
@@ -70,7 +70,11 @@
                 // move into the READY state and the PUK lock keyguard should be removed.
                 case READY: {
                     KeyguardUpdateMonitor.getInstance(getContext()).reportSimUnlocked(mSubId);
-                    mCallback.dismiss(true, KeyguardUpdateMonitor.getCurrentUser());
+                    // mCallback can be null if onSimStateChanged callback is called when keyguard
+                    // isn't active.
+                    if (mCallback != null) {
+                        mCallback.dismiss(true, KeyguardUpdateMonitor.getCurrentUser());
+                    }
                     break;
                 }
                 default: