fix: resolve tab close and a startup crash on macOS that occured

PrepareForMacUser.md

@@ -8,7 +8,7 @@ Then reactivate your virtual environment:
 deactivate
 source venv/bin/activate
 ```
-#### If you are using Apple Silicon (M1/M2/M3)
+#### If you are using Apple Silicon (M1/M2/M3/M4)
 
 Run:
 ```bash
@@ -44,4 +44,4 @@ export DYLD_FALLBACK_LIBRARY_PATH=/usr/local/lib
 Save the file, then run:
 ```bash
 source ~/.zshrc
-```
+```

app.py

@@ -1,55 +1,64 @@
-import tkinter as tk
-from markdown_reader.ui import MarkdownReader
-import sys
 import os
+import sys
+import tkinter as tk
+
 import ttkbootstrap as ttkb
 from ttkbootstrap.constants import *
 
+from markdown_reader.ui import MarkdownReader
+
 
 def handle_open_file(event):
     """
     Handles file open events from macOS.
 
-    :param event event: The open file event from macOS. 
+    :param event event: The open file event from macOS.
     """
-    
+
     file_path = event
-    if os.path.isfile(file_path) and file_path.lower().endswith(('.md', '.markdown', '.html', '.htm', '.pdf')):
+    if os.path.isfile(file_path) and file_path.lower().endswith(
+        (".md", ".markdown", ".html", ".htm", ".pdf")
+    ):
         app.load_file(file_path)
 
 
 if __name__ == "__main__":
     try:
         from tkinterdnd2 import TkinterDnD
+
         root = TkinterDnD.Tk()
-        
+
         # Apply ttkbootstrap theme to TkinterDnD window
         app_style = ttkb.Style(theme="darkly")
-        
+
         print("TkinterDnD enabled - Drag and drop support available")
-    except ImportError as e:
+    except (ImportError, RuntimeError) as e:
         print(f"   Warning: tkinterdnd2 not installed, drag-and-drop will be disabled")
         print(f"   Error: {e}")
         root = ttkb.Window(themename="darkly")
-    
+
     # Ensure window is resizable
     root.resizable(width=True, height=True)
 
     app = MarkdownReader(root)
-    
+
     # Handle file open events from macOS Finder
-    root.createcommand("::tk::mac::OpenDocument", lambda *args: handle_open_file(args[0]))
+    root.createcommand(
+        "::tk::mac::OpenDocument", lambda *args: handle_open_file(args[0])
+    )
 
     # Handle file opening from command line
     if len(sys.argv) > 1:
         for arg in sys.argv[1:]:
             # Skip macOS system parameters (process serial number)
-            if arg.startswith('-psn'):
+            if arg.startswith("-psn"):
                 continue
             # Convert to absolute path
             file_path = os.path.abspath(arg)
-            if os.path.isfile(file_path) and file_path.lower().endswith(('.md', '.markdown', '.html', '.htm', '.pdf')):
+            if os.path.isfile(file_path) and file_path.lower().endswith(
+                (".md", ".markdown", ".html", ".htm", ".pdf")
+            ):
                 app.load_file(file_path)
                 break  # Only open the first file
 
-    root.mainloop()
+    root.mainloop()