Allow ViewCB's generic M parameter to be of any type

... and begin implementing CachedViewCB#EMPTY_MODEL support

src/main/java/io/bitsquare/gui/CachedViewCB.java

@@ -24,21 +24,34 @@ import java.util.ResourceBundle;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 /**
  * If caching is used for loader we use the CachedViewController for turning the controller into sleep mode if not
  * active and awake it at reactivation.
  */
-public class CachedViewCB<M extends Activatable & Model> extends ViewCB<M> {
+public class CachedViewCB<M extends Activatable> extends ViewCB<M> {
     private static final Logger log = LoggerFactory.getLogger(CachedViewCB.class);
 
+    protected static final Activatable EMPTY_MODEL = new Activatable() {
+        @Override
+        public void activate() {
+        }
+
+        @Override
+        public void deactivate() {
+        }
+    };
+
     public CachedViewCB(M model) {
-        super(model);
+        super(checkNotNull(model, "Model must not be null"));
     }
 
     public CachedViewCB() {
-        this(null);
+        this((M) EMPTY_MODEL);
     }
 
+
     /**
      * Get called form GUI framework when the UI is ready.
      * In caching controllers the initialize is only used for static UI setup.

src/main/java/io/bitsquare/gui/ViewCB.java

@@ -31,7 +31,7 @@ import org.slf4j.LoggerFactory;
 /**
  * Non caching version for code behind classes using the PM pattern
  */
-public class ViewCB<M extends Model> implements Initializable {
+public class ViewCB<M> implements Initializable {
 
     private static final Logger log = LoggerFactory.getLogger(ViewCB.class);
 

src/main/java/io/bitsquare/gui/main/funds/FundsViewCB.java

@@ -51,7 +51,6 @@ public class FundsViewCB extends CachedViewCB {
 
     @Inject
     FundsViewCB(ViewLoader viewLoader, Navigation navigation) {
-        super();
         this.viewLoader = viewLoader;
         this.navigation = navigation;
     }