fix: 修复打开多个应用窗口，在任务栏预览中无法关闭窗口的问题

因为鼠标在进入预览窗口时，触发了requestCancelPreviewWindow信号，将预览窗口的指针给置为空，当关闭应用时，无法隐藏并刷新
预览窗口

Log: 修复打开多个应用窗口，在任务栏预览中无法关闭窗口的问题
Bug: https://pms.uniontech.com/zentao/bug-view-40031.html
Change-Id: I00e08d24409d346fd44f16cb5f3813694e5daaf8
Reviewed-on: http://gerrit.uniontech.com/c/dde-dock/+/974
Reviewed-by: <mailman@uniontech.com>
Reviewed-by: lizhongming <lizhongming@uniontech.com>
Tested-by: <mailman@uniontech.com>

frame/item/components/previewcontainer.cpp

@@ -224,7 +224,6 @@ void PreviewContainer::onSnapshotClicked(const WId wid)
     Q_EMIT requestActivateWindow(wid);
     m_needActivate = true;
     m_waitForShowPreviewTimer->stop();
-    requestCancelPreviewWindow();
     requestHidePopup();
 }
 
@@ -248,8 +247,6 @@ void PreviewContainer::previewEntered(const WId wid)
 
     m_floatingPreview->trackWindow(snap);
 
-    requestCancelPreviewWindow();
-
     if (m_waitForShowPreviewTimer->isActive()) {
         return;
     }