fix: fix "Cannot read property 'kill' of undefined" error (#822)

`Cannot read property 'kill' of undefined` is thrown when `ctrl+c` is clicked too early when webpack process is started but still not persisted

Author: Fatme

Diff for: lib/compiler.js

@@ -173,7 +173,9 @@ function logSnapshotWarningMessage($logger) {
 
 function stopWebpackForPlatform(platform) {
     const webpackProcess = webpackProcesses[platform];
-    webpackProcess.kill("SIGINT");
-    delete webpackProcesses[platform];
+    if (webpackProcess) {
+        webpackProcess.kill("SIGINT");
+        delete webpackProcesses[platform];
+    }
 }