sitessetuppresenter: check for add dialog callback nullity

somehow the dialog is first detached from the window, and then
the button click is propagated.
not sure how this can happen, because we have overridden the button
press dismiss behavior.

Clover/app/src/main/java/org/floens/chan/core/presenter/SitesSetupPresenter.java

@@ -88,12 +88,16 @@ public class SitesSetupPresenter {
             @Override
             public void onSiteAdded(Site site) {
                 siteAdded(site);
-                addCallback.dismissDialog();
+                if (addCallback != null) {
+                    addCallback.dismissDialog();
+                }
             }
 
             @Override
             public void onSiteAddFailed(String message) {
-                addCallback.showAddError(message);
+                if (addCallback != null) {
+                    addCallback.showAddError(message);
+                }
             }
         });
     }