Fixed potential crash when removing a tileset

When a tileset is removed its view was deleted before the tab was
removed from the tab bar. This could lead to the case where
currentTilesetDocument could return the wrong tileset or crash, even
though currentTilesetView had returned a valid view.

With this change, we make these functions behave consistently, which
hopefully avoids such issues.

src/tiled/tilesetdock.cpp

@@ -935,7 +935,7 @@ void TilesetDock::setCurrentTileset(const SharedTileset &tileset)
 
 SharedTileset TilesetDock::currentTileset() const
 {
-    const int index = mTabBar->currentIndex();
+    const int index = mViewStack->currentIndex();
     if (index == -1)
         return {};
 
@@ -944,7 +944,7 @@ SharedTileset TilesetDock::currentTileset() const
 
 TilesetDocument *TilesetDock::currentTilesetDocument() const
 {
-    const int index = mTabBar->currentIndex();
+    const int index = mViewStack->currentIndex();
     if (index == -1)
         return nullptr;