[KOE-133] Added checks for null accounts and explorers, which happens when an email is sent through another application.

src/AcaciaZPushPlugin/AcaciaZPushPlugin/Stubs/OutlookWrappers/AddInWrapper.cs

@@ -251,7 +251,10 @@ namespace Acacia.Stubs.OutlookWrappers
 
         public IExplorer GetActiveExplorer()
         {
-            return new ExplorerWrapper(_app.ActiveExplorer());
+            NSOutlook.Explorer explorer = _app.ActiveExplorer();
+            if (explorer == null)
+                return null;
+            return new ExplorerWrapper(explorer);
         }
 
         public IInspectors GetInspectors()

src/AcaciaZPushPlugin/AcaciaZPushPlugin/ZPush/FolderRegistration.cs

@@ -74,6 +74,8 @@ namespace Acacia.ZPush
             // TODO: cache folder id per store
             using (IStore store = folder.GetStore())
             {
+                if (store == null)
+                    return false;
                 return folder.EntryID == store.GetDefaultFolderId(_folder);
             }
         }

src/AcaciaZPushPlugin/AcaciaZPushPlugin/ZPush/ZPushAccounts.cs

@@ -137,6 +137,8 @@ namespace Acacia.ZPush
 
         public ZPushAccount GetAccount(IStore store)
         {
+            if (store == null)
+                return null;
             ZPushAccount value = null;
             _accountsByStoreId.TryGetValue(store.StoreID, out value);
             return value;

src/AcaciaZPushPlugin/AcaciaZPushPlugin/ZPush/ZPushWatcher.cs

@@ -52,15 +52,17 @@ namespace Acacia.ZPush
             Accounts = new ZPushAccounts(this, addIn);
 
             // Need to keep a link to keep receiving events
+            // Might be null when starting a compose window only
             _explorer = _addIn.GetActiveExplorer();
-            _explorer.SelectionChange += Explorer_SelectionChange;
+            if (_explorer != null)
+                _explorer.SelectionChange += Explorer_SelectionChange;
         }
 
         protected override void DoRelease()
         {
             Accounts.Dispose();
             Sync.Dispose();
-            _explorer.Dispose();
+            _explorer?.Dispose();
         }
 
         /// <summary>