fix: compatibility with webpack v4

Author: alexander-akait

lib/Server.js

@@ -1033,6 +1033,9 @@ class Server {
 
       providePlugin.apply(compiler);
 
+      // TODO remove after drop webpack v4 support
+      compiler.options.plugins = compiler.options.plugins || [];
+
       if (
         this.options.hot &&
         !compiler.options.plugins.find(