fix: suppress FileSelector callbacks during construction

During _build_config_panel(), set_relative_path() triggered callbacks
that called _get_selected_files() before all selectors were created,
causing AttributeError.

Fix: FileSelector now has _suppress_callback flag. Set True during
construction, then False after all selectors are created. Finally
call _auto_save_config() once at the end of the panel build.

src/gui/main_window.py

@@ -218,15 +218,20 @@ class MainWindow(ctk.CTk):
         object's perspective); the FileSelector stores both the
         absolute path (for internal use) and the relative path
         (for saving to YAML).
+
+        All attribute accesses are defensive — widgets may not exist
+        yet during construction (e.g. _port_var created in _build_serial).
         """
         if not self._config:
             return
-        # Sync IP address
+        # Sync IP address (may not exist yet during _build_config_panel)
         if self._ip_string_var:
             self._config.zynq_ip = self._ip_string_var.get()
         # Sync Xilinx Kit path
+        if hasattr(self, '_xilinx_path_var'):
             self._config.xilinx_path = self._xilinx_path_var.get()
-        # Sync serial port
+        # Sync serial port (may not exist yet during _build_config_panel)
+        if hasattr(self, '_port_var'):
             self._config.serial_port = self._port_var.get()
         # Sync file paths from FileSelectors
         files = self._get_selected_files()
@@ -245,6 +250,8 @@ class MainWindow(ctk.CTk):
                 if abs_path:
                     self._config._config_path  # ensure _config_path is set
                     setattr(self._config, attr, self._config._relative_path(abs_path))
+        # Sync erase checkbox (may not exist yet)
+        if hasattr(self, '_erase_cb_var'):
             self._config.erase_all = self._erase_cb_var.get()
 
     def _auto_save_config(self) -> None:
@@ -586,6 +593,7 @@ class MainWindow(ctk.CTk):
             file_types=[("ELF files", "*.elf"), ("All files", "*")],
             callback=self._on_file_selected,
         )
+        self._fsbl_selector._suppress_callback = True
         if self._config.fsbl_elf_path:
             resolved = self._config.resolve_path(self._config.fsbl_elf_path)
             self._fsbl_selector.set_relative_path(self._config.fsbl_elf_path)
@@ -599,6 +607,7 @@ class MainWindow(ctk.CTk):
             file_types=[("BIT files", "*.bit"), ("All files", "*")],
             callback=self._on_file_selected,
         )
+        self._bit_selector._suppress_callback = True
         if self._config.bootloader_bit_path:
             resolved = self._config.resolve_path(self._config.bootloader_bit_path)
             self._bit_selector.set_relative_path(self._config.bootloader_bit_path)
@@ -612,6 +621,7 @@ class MainWindow(ctk.CTk):
             file_types=[("ELF files", "*.elf"), ("All files", "*")],
             callback=self._on_file_selected,
         )
+        self._elf_selector._suppress_callback = True
         if self._config.bootloader_elf_path:
             resolved = self._config.resolve_path(self._config.bootloader_elf_path)
             self._elf_selector.set_relative_path(self._config.bootloader_elf_path)
@@ -625,6 +635,7 @@ class MainWindow(ctk.CTk):
             file_types=[("BIN files", "*.bin"), ("All files", "*")],
             callback=self._on_file_selected,
         )
+        self._bootloader_bin_selector._suppress_callback = True
         if self._config.bootloader_bin_path:
             resolved = self._config.resolve_path(self._config.bootloader_bin_path)
             self._bootloader_bin_selector.set_relative_path(self._config.bootloader_bin_path)
@@ -660,12 +671,21 @@ class MainWindow(ctk.CTk):
             file_types=[("BIN files", "*.bin"), ("All files", "*")],
             callback=self._on_file_selected,
         )
+        self._firmware_bin_selector._suppress_callback = True
         if self._config.firmware_bin_path:
             resolved = self._config.resolve_path(self._config.firmware_bin_path)
             self._firmware_bin_selector.set_relative_path(self._config.firmware_bin_path)
             self._firmware_bin_selector.set_path(str(resolved))
         self._firmware_bin_selector.grid(row=8, column=0, sticky="nsew", padx=PADDING, pady=PADDING_SMALL)
 
+        # Enable callbacks now that all selectors are created
+        for sel in (self._fsbl_selector, self._bit_selector, self._elf_selector,
+                     self._bootloader_bin_selector, self._firmware_bin_selector):
+            sel._suppress_callback = False
+
+        # Now sync and save config (all selectors exist)
+        self._auto_save_config()
+
         # Save button
         save_btn = ctk.CTkButton(
             panel,

src/gui/widgets.py

@@ -528,6 +528,7 @@ class FileSelector(ctk.CTkFrame):
         self._full_path: str = ""           # absolute path (internal)
         self._relative_path: str = ""       # relative path (synced to Config)
         self._display_text = ctk.StringVar(value="")
+        self._suppress_callback: bool = False  # True during construction
 
         self._create_widgets(label_text)
 
@@ -590,7 +591,7 @@ class FileSelector(ctk.CTkFrame):
         """
         self._full_path = path
         self._display_text.set(os.path.basename(path) if path else "")
-        if self._callback:
+        if self._callback and not self._suppress_callback:
             self._callback(path)
 
     def set_relative_path(self, path: str) -> None:
@@ -606,7 +607,7 @@ class FileSelector(ctk.CTkFrame):
         if path:
             self._full_path = path  # caller should resolve before calling
         self._display_text.set(os.path.basename(path) if path else "")
-        if self._callback:
+        if self._callback and not self._suppress_callback:
             self._callback(path)
 
     def get_path(self) -> str: