Corrected NPE in the engine's pause method

Author: rhauch

build.properties

@@ -1,4 +1,4 @@
-strongback.version=1.2.0-Beta1
+strongback.version=1.2.0-Beta2
 #
 # The build will download a specific version of the WPILib given by the following URL
 # and install it into the 'libs/wpilib' folder. To use a different version of WPILib,

strongback/src/org/strongback/Strongback.java

@@ -1271,8 +1271,10 @@ public boolean isRunning() {
             return running.get();
         }
 
-        public void pause() {
-            executor.stop();
+        public synchronized void pause() {
+            if (running.get()) {
+                executor.stop();
+            }
         }
 
         /**