sheet_viewer: try forcing garbage collect on pdf

lib/sheet_viewer_page.dart

@@ -21,12 +21,19 @@ class _SheetViewerPageState extends State<SheetViewerPage> {
   int page = 1;
   int numPages = 1;
   bool isFullscreen = false;
+  PdfDocument? document;
 
   @override
   void initState() {
     super.initState();
   }
 
+  @override
+  void dispose() {
+    document?.dispose(); // Make sure document gets garbage collected
+    super.dispose();
+  }
+
   Future<PdfDocument> loadPdf() async {
     var file = await widget.apiClient.getPdfFileCached(widget.sheet.uuid);
     if (file == null) {
@@ -63,9 +70,9 @@ class _SheetViewerPageState extends State<SheetViewerPage> {
           future: loadPdf(),
           builder: (BuildContext context, AsyncSnapshot<PdfDocument> snapshot) {
             if (snapshot.hasData) {
-              var document = snapshot.data;
+              document = snapshot.data;
               if (document != null) {
-                numPages = document.pages.length;
+                numPages = document!.pages.length;
               }
               return GestureDetector(
                 onTapUp: (TapUpDetails details) {