feat: typecheck handler to be a function

src/event.ts

@@ -37,6 +37,10 @@ export function defineLazyEventHandler (factory: LazyEventHandler): EventHandler
     if (_resolved) { return Promise.resolve(_resolved) }
     if (!_promise) {
       _promise = Promise.resolve(factory()).then((r: any) => {
+        const handler = r.default || r
+        if (typeof handler !== 'function') {
+          throw new TypeError('Invalid lazy handler result. It should be a function:', handler)
+        }
         _resolved = toEventHandler(r.default || r)
         return _resolved
       })
@@ -61,6 +65,9 @@ export function toEventHandler (handler: CompatibilityEventHandler): EventHandle
   if (isEventHandler(handler)) {
     return handler
   }
+  if (typeof handler !== 'function') {
+    throw new TypeError('Invalid handler. It should be a function:', handler)
+  }
   return defineEventHandler((event) => {
     return callHandler(handler, event.req as IncomingMessage, event.res) as Promise<H3Response>
   })