Fix null error on shutdown (#1072)

Author: grouma

dwds/CHANGELOG.md

@@ -7,6 +7,7 @@
 - Allow reading metadata generated by `dev_compiler` from file to supply
   module information to `Dwds`.
 - Hide JavaScript type errors when hovering over text in the debugger.
+- Fix an issue where reusing a connection could cause a null error.
 
 **Breaking Changes:**
 - Require access to the `.ddc_merged_metadata` file.

dwds/lib/src/handlers/dev_handler.dart

@@ -388,7 +388,7 @@ class DevHandler {
     if (canReuseConnection) {
       // Disconnect any old connection (eg. those in the keep-alive waiting
       // state when reloading the page).
-      existingAppConection.shutDown();
+      existingAppConection?.shutDown();
       services.chromeProxyService?.destroyIsolate();
 
       // Reconnect to existing service.