fix memory leak (#1865)

HMCL/src/main/java/org/jackhuang/hmcl/game/LauncherHelper.java

@@ -756,6 +756,7 @@ public final class LauncherHelper {
                             Controllers.getStage().close();
                             Controllers.shutdown();
                             Schedulers.shutdown();
+                            System.gc();
                         }
                     });
                     break;

HMCL/src/main/java/org/jackhuang/hmcl/ui/Controllers.java

@@ -315,6 +315,8 @@ public final class Controllers {
         rootPage = null;
         versionPage = null;
         gameListPage = null;
+        downloadPage = null;
+        accountListPage = null;
         settingsPage = null;
         decorator = null;
         stage = null;