strictNullChecks: fix plugin runtime workers

- Add non-null assertions for worker and stdio access
- Add non-null assertions for kill() undefined assignments

server/src/plugin/runtime/custom-worker.ts

@@ -52,8 +52,8 @@ export class CustomRuntimeWorker extends ChildProcessWorker {
     }
 
     setupRpcPeer(peer: RpcPeer): void {
-        const peerin = this.worker.stdio[3] as Writable;
-        const peerout = this.worker.stdio[4] as Readable;
+        const peerin = this.worker!.stdio[3]! as Writable;
+        const peerout = this.worker!.stdio[4]! as Readable;
 
         const serializer = this.serializer = createRpcDuplexSerializer(peerin);
         serializer.setupRpcPeer(peer);

server/src/plugin/runtime/node-fork-worker.ts

@@ -77,7 +77,7 @@ export class NodeForkWorker extends ChildProcessWorker {
     }
 
     setupRpcPeer(peer: RpcPeer): void {
-        this.worker.on('message', (message, sendHandle) => {
+        this.worker!.on('message', (message, sendHandle) => {
             if ((message as any).type && sendHandle) {
                 peer.handleMessage(message as any, {
                     sendHandle,

server/src/plugin/runtime/node-thread-worker.ts

@@ -104,8 +104,8 @@ export class NodeThreadWorker extends EventEmitter implements RuntimeWorker {
             return;
         this.worker.terminate();
         this.port.close();
-        this.port = undefined;
-        this.worker = undefined;
+        this.port = undefined!;
+        this.worker = undefined!;
     }
 
     send(message: RpcMessage, reject?: (e: Error) => void, serializationContext?: any): void {