fix box.json getting emptied on Ctrl-C (#501)

Author: rforced

src/Commands/BuildCommand.php

@@ -208,13 +208,15 @@ function () use ($configFile, $config) {
 
     private function clear(): void
     {
-        File::put($this->app->configPath('app.php'), self::$config);
-
-        File::put($this->app->basePath('box.json'), self::$box);
-
-        self::$config = null;
+        if (self::$config !== null) {
+            File::put($this->app->configPath('app.php'), self::$config);
+            self::$config = null;
+        }
 
-        self::$box = null;
+        if (self::$box !== null) {
+            File::put($this->app->basePath('box.json'), self::$box);
+            self::$box = null;
+        }
     }
 
     /**