fix: handle middleware teardown in plugin mode

lib/Server.js

@@ -3660,19 +3660,23 @@ class Server {
       );
 
       if (this.middleware) {
-        await /** @type {Promise<void>} */ (
-          new Promise((resolve, reject) => {
-            /** @type {import("webpack-dev-middleware").API<Request, Response>} */
-            (this.middleware).close((error) => {
-              if (error) {
-                reject(error);
-                return;
-              }
+        // In plugin mode the middleware has no `watching` of its own to close
+        // (the host's `compiler.close()` handles it).
+        if (!this.isPlugin) {
+          await /** @type {Promise<void>} */ (
+            new Promise((resolve, reject) => {
+              /** @type {import("webpack-dev-middleware").API<Request, Response>} */
+              (this.middleware).close((error) => {
+                if (error) {
+                  reject(error);
+                  return;
+                }
 
-              resolve();
-            });
-          })
-        );
+                resolve();
+              });
+            })
+          );
+        }
 
         this.middleware = undefined;
       }

test/e2e/api-plugin.test.js

@@ -109,6 +109,23 @@ describe("API (plugin)", () => {
     stopSpy.mockRestore();
   });
 
+  it("should tear down without error when the compiler is closed", async () => {
+    // In plugin mode the host owns `watching`, so `stop()` must not call
+    // `middleware.close()` — otherwise `compiler.close` surfaces a TypeError.
+    const compiler = webpack(config);
+    const server = new Server({ port });
+
+    server.apply(compiler);
+
+    await compile(compiler, port);
+
+    const closeError = await new Promise((resolve) => {
+      compiler.close((error) => resolve(error));
+    });
+
+    expect(closeError).toBeFalsy();
+  });
+
   it("should stay passive in build mode (compiler.run)", async () => {
     // The shared fixture writes output to "/", which would be unwritable
     // outside of webpack-dev-middleware's in-memory FS. Use a tmp dir so the