Null phone check when checking if radio is off for thermal mitigation.

Bug: 193858761
Test: https://android-build.googleplex.com/builds/abtd/run/L07000000950619836 and attempt to reproduce

Change-Id: I52a57517cf2f13200f5421d5bbfe2fff9da424da
diff --git a/src/com/android/services/telephony/DisconnectCauseUtil.java b/src/com/android/services/telephony/DisconnectCauseUtil.java
index 9c89e9e..ffd9f63 100644
--- a/src/com/android/services/telephony/DisconnectCauseUtil.java
+++ b/src/com/android/services/telephony/DisconnectCauseUtil.java
@@ -792,7 +792,7 @@
 
     private static boolean isRadioOffForThermalMitigation(int phoneId) {
         Phone phone = PhoneFactory.getPhone(phoneId);
-        return phone.isRadioOffForThermalMitigation();
+        return phone == null ? false : phone.isRadioOffForThermalMitigation();
     }
 
     /**