fix: errors when `themeColor.fixed` is set to `true` (#160)

Prevent null errors in Navbar.astro by verifying DOM elements before attaching event listeners

src/components/Navbar.astro

@@ -84,21 +84,31 @@ function switchTheme() {
 
 function loadButtonScript() {
     let switchBtn = document.getElementById("scheme-switch");
-    switchBtn!.addEventListener("click", function () {
+    if (switchBtn) {
+        switchBtn.addEventListener("click", function () {
             switchTheme()
         });
+    }
 
     let settingBtn = document.getElementById("display-settings-switch");
-    settingBtn!.addEventListener("click", function () {
+    if (settingBtn) {
+        settingBtn.addEventListener("click", function () {
             let settingPanel = document.getElementById("display-setting");
-        settingPanel!.classList.toggle("float-panel-closed");
+            if (settingPanel) {
+                settingPanel.classList.toggle("float-panel-closed");
+            }
         });
+    }
 
     let menuBtn = document.getElementById("nav-menu-switch");
-    menuBtn!.addEventListener("click", function () {
+    if (menuBtn) {
+        menuBtn.addEventListener("click", function () {
             let menuPanel = document.getElementById("nav-menu-panel");
-        menuPanel!.classList.toggle("float-panel-closed");
+            if (menuPanel) {
+                menuPanel.classList.toggle("float-panel-closed");
+            }
         });
+    }
 }
 
 loadButtonScript();