diff --git a/eslint.config.mjs b/eslint.config.mjs
index 71732a6..a2aba3b 100644
--- a/eslint.config.mjs
+++ b/eslint.config.mjs
@@ -8,4 +8,15 @@ const urlConstantsOverride = {
   },
 };
 
-export default [...config, urlConstantsOverride];
+// Forbid non-null assertions (`value!`). The `!` operator silently tells the
+// type checker a value can't be null/undefined without any runtime guarantee,
+// so a wrong assumption surfaces as a runtime crash instead of a compile error.
+// `eslint-config-agent` does not ship this rule. Prefer an explicit guard,
+// optional chaining, or narrowing instead.
+const noNonNullAssertion = {
+  rules: {
+    "@typescript-eslint/no-non-null-assertion": "error",
+  },
+};
+
+export default [...config, urlConstantsOverride, noNonNullAssertion];
diff --git a/src/app/open/use-auto-close.ts b/src/app/open/use-auto-close.ts
index bb1ee53..11ff353 100644
--- a/src/app/open/use-auto-close.ts
+++ b/src/app/open/use-auto-close.ts
@@ -22,8 +22,7 @@ export function useAutoClose(active: boolean) {
     intervalRef.current = setInterval(() => {
       setCountdown((prev) => {
         if (prev <= 1) {
-          clearInterval(intervalRef.current!);
-          intervalRef.current = null;
+          clearTimer();
           window.close();
           return 0;
         }