fix: 防御性检查补全

- openFileInTab 增加 isDestroyed() 前置检查
- open-file handler 增加 isDestroyed() 检查
- window:forceClose 增加 isDestroyed() 检查
- close handler 使用 isClosing 防止重入
- cancelClose 重置 isClosing 允许再次关闭

main.js

@@ -7,6 +7,7 @@ let activeFilePath = null; // Currently active tab's file path
 let pendingFilePath = null;
 let fileWatcher = null;
 let closeTimeout = null;
+let isClosing = false; // Prevent re-entrant close
 
 // ===== Single Instance Lock =====
 const gotTheLock = app.requestSingleInstanceLock();
@@ -17,7 +18,7 @@ if (!gotTheLock) {
   app.on('second-instance', (event, commandLine, workingDirectory) => {
     // A second instance was launched (e.g. double-clicking a .md file)
     // Focus the existing window and open the file in it
-    if (mainWindow) {
+    if (mainWindow && !mainWindow.isDestroyed()) {
       if (mainWindow.isMinimized()) mainWindow.restore();
       mainWindow.focus();
 
@@ -76,6 +77,8 @@ function createWindow() {
 
   // Unsaved changes warning on close
   mainWindow.on('close', (e) => {
+    if (isClosing) return; // Prevent re-entrant close
+    isClosing = true;
     e.preventDefault();
     try {
       mainWindow.webContents.send('window:confirmClose');
@@ -140,6 +143,7 @@ function readFileContent(filePath) {
 
 // Open file in a new tab (renderer manages tabs)
 function openFileInTab(filePath) {
+  if (!mainWindow || mainWindow.isDestroyed()) return;
   try {
     const content = readFileContent(filePath);
     activeFilePath = filePath;
@@ -276,8 +280,10 @@ ipcMain.handle('window:forceClose', () => {
     closeTimeout = null;
   }
   stopWatching();
+  if (mainWindow && !mainWindow.isDestroyed()) {
     mainWindow.removeAllListeners('close');
     mainWindow.close();
+  }
 });
 
 ipcMain.handle('window:cancelClose', () => {
@@ -285,6 +291,7 @@ ipcMain.handle('window:cancelClose', () => {
     clearTimeout(closeTimeout);
     closeTimeout = null;
   }
+  isClosing = false; // Reset so close can be triggered again
 });
 
 // Command line file (first instance)
@@ -296,9 +303,9 @@ function getCommandLineFile() {
 app.whenReady().then(() => {
   app.on('open-file', (event, filePath) => {
     event.preventDefault();
-    if (mainWindow && mainWindow.webContents.isLoading()) {
+    if (mainWindow && !mainWindow.isDestroyed() && mainWindow.webContents.isLoading()) {
       pendingFilePath = filePath;
-    } else if (mainWindow) {
+    } else if (mainWindow && !mainWindow.isDestroyed()) {
       openFileInTab(filePath);
     } else {
       pendingFilePath = filePath;