Fix watch exiting if no plugin

ci/dev/watch.ts

@@ -37,7 +37,9 @@ class Watcher {
 
     const vscode = cp.spawn("yarn", ["watch"], { cwd: this.vscodeSourcePath })
     const tsc = cp.spawn("tsc", ["--watch", "--pretty", "--preserveWatchOutput"], { cwd: this.rootPath })
-    const plugin = cp.spawn("yarn", ["build", "--watch"], { cwd: process.env.PLUGIN_DIR })
+    const plugin = process.env.PLUGIN_DIR
+      ? cp.spawn("yarn", ["build", "--watch"], { cwd: process.env.PLUGIN_DIR })
+      : undefined
     const bundler = this.createBundler()
 
     const cleanup = (code?: number | null): void => {
@@ -49,9 +51,11 @@ class Watcher {
       tsc.removeAllListeners()
       tsc.kill()
 
+      if (plugin) {
         Watcher.log("killing plugin")
         plugin.removeAllListeners()
         plugin.kill()
+      }
 
       if (server) {
         Watcher.log("killing server")
@@ -74,10 +78,12 @@ class Watcher {
       Watcher.log("tsc terminated unexpectedly")
       cleanup(code)
     })
+    if (plugin) {
       plugin.on("exit", (code) => {
         Watcher.log("plugin terminated unexpectedly")
         cleanup(code)
       })
+    }
     const bundle = bundler.bundle().catch(() => {
       Watcher.log("parcel watcher terminated unexpectedly")
       cleanup(1)
@@ -91,7 +97,9 @@ class Watcher {
 
     vscode.stderr.on("data", (d) => process.stderr.write(d))
     tsc.stderr.on("data", (d) => process.stderr.write(d))
+    if (plugin) {
       plugin.stderr.on("data", (d) => process.stderr.write(d))
+    }
 
     // From https://github.com/chalk/ansi-regex
     const pattern = [
@@ -151,6 +159,7 @@ class Watcher {
       }
     })
 
+    if (plugin) {
       onLine(plugin, (line, original) => {
         // tsc outputs blank lines; skip them.
         if (line !== "") {
@@ -161,6 +170,7 @@ class Watcher {
         }
       })
     }
+  }
 
   private createBundler(out = "dist"): Bundler {
     return new Bundler(