Make sure it does not throw in SSR (#137)

Make sure it does not throw in a SSR (Server Side Rendering) situation

Author: landabaso

src/ResizeSensor.js

@@ -14,6 +14,10 @@
     }
 }(this, function () {
 
+    //Make sure it does not throw in a SSR (Server Side Rendering) situation
+    if (typeof window === "undefined") {
+        return null;
+    }
     // Only used for the dirty checking, so the event callback count is limted to max 1 call per fps per sensor.
     // In combination with the event based resize sensor this saves cpu time, because the sensor is too fast and
     // would generate too many unnecessary events.