Clean up the tmc-core singleton implementaion

Author: jamo

tmc-plugin/src/fi/helsinki/cs/tmc/model/TmcCoreSingleton.java

@@ -5,12 +5,12 @@
 
 public class TmcCoreSingleton {
 
-    private static TmcCore defaultInstance;
-    
+    private static final TmcCore defaultInstance = new TmcCore(NbTmcSettings.getDefault());
+
+    /**
+     * Returns singleton instance of the {@link TmcCore}.
+     */
     public static TmcCore getInstance() {
-        if (defaultInstance == null) {
-            defaultInstance = new TmcCore(NbTmcSettings.getDefault());
-        }
         return defaultInstance;
     }
-}
+}