Add a null check for onSuccess and onError Callbacks (#96)

On a RosieUseCase, success and error callbacks are stored on WeakReferences
fields. These callbacks could be recycled before they are exicited
by the use cases to return the result of them, especially if the task are
really heavy, so it is necessary a null check before use them.

rosie/src/main/java/com/karumi/rosie/domain/usecase/RosieUseCase.java

@@ -52,15 +52,18 @@ public void setCallbackScheduler(CallbackScheduler callbackScheduler) {
    * this method returns the response in the UI Thread.
    */
   protected void notifySuccess(Object... values) {
-    Method[] methodsArray = onSuccessCallback.get().getClass().getMethods();
-    if (methodsArray.length > 0) {
-      Method methodToInvoke =
-          UseCaseFilter.filterValidMethodArgs(values, methodsArray, Success.class);
-      invokeMethodInTheCallbackScheduler(methodToInvoke, values);
-    } else {
-      throw new IllegalStateException(
-          "The OnSuccessCallback instance configured has no methods annotated with the "
-              + "@Success annotation.");
+    OnSuccessCallback successCallback = onSuccessCallback.get();
+    if (successCallback != null) {
+      Method[] methodsArray = successCallback.getClass().getMethods();
+      if (methodsArray.length > 0) {
+        Method methodToInvoke =
+            UseCaseFilter.filterValidMethodArgs(values, methodsArray, Success.class);
+        invokeMethodInTheCallbackScheduler(methodToInvoke, values);
+      } else {
+        throw new IllegalStateException(
+            "The OnSuccessCallback instance configured has no methods annotated with the "
+                + "@Success annotation.");
+      }
     }
   }
 
@@ -74,7 +77,10 @@ protected void notifySuccess(Object... values) {
    */
 
   protected void notifyError(final Error error) throws ErrorNotHandledException {
-    onErrorCallback.get().onError(error);
+    OnErrorCallback errorCallback = onErrorCallback.get();
+    if (errorCallback != null) {
+      errorCallback.onError(error);
+    }
   }
 
   /**
@@ -104,12 +110,12 @@ void setOnErrorCallback(OnErrorCallback onErrorCallback) {
   private void invokeMethodInTheCallbackScheduler(final Method methodToInvoke,
       final Object[] values) {
     if (onSuccessCallback != null) {
-      OnSuccessCallback callback = onSuccessCallback.get();
+      final OnSuccessCallback callback = onSuccessCallback.get();
       if (callback != null) {
         getCallbackScheduler().post(new Runnable() {
           @Override public void run() {
             try {
-              methodToInvoke.invoke(onSuccessCallback.get(), values);
+              methodToInvoke.invoke(callback, values);
             } catch (Exception e) {
               throw new RuntimeException("Internal error invoking the success object", e);
             }