Fix potential NPE

Author: bilal-alsharifi

base/src/main/java/com/smartdevicelink/managers/screen/BaseSoftButtonManager.java

@@ -223,8 +223,10 @@ public void dispose() {
         softButtonCapabilities = null;
 
         // Cancel the operations
-        transactionQueue.close();
-        transactionQueue = null;
+        if (transactionQueue != null) {
+            transactionQueue.close();
+            transactionQueue = null;
+        }
 
         // Remove listeners
         internalInterface.removeOnRPCNotificationListener(FunctionID.ON_HMI_STATUS, onHMIStatusListener);