Prevented a potential null access

assets/scripts/init.toy

@@ -1,5 +1,6 @@
 import engine;
 
+//this function must always be called, or the engine won't run
 initWindow("Airport Game", 800, 600, false);
 
 loadRootNode("assets/scripts/root.toy");

core/engine.c

@@ -146,7 +146,9 @@ void freeEngine() {
 
 static void execStep() {
 	//call onStep
-	callEngineNode(engine.rootNode, &engine.interpreter, "onStep");
+	if (engine.rootNode != NULL) {
+		callEngineNode(engine.rootNode, &engine.interpreter, "onStep");
+	}
 
 	//poll events
 	SDL_Event event;

core/lib_engine.c

@@ -191,7 +191,9 @@ static int nativeLoadRootNode(Interpreter* interpreter, LiteralArray* arguments)
 	size_t originalSize = engine.interpreter.length;
 	int originalCount = engine.interpreter.count;
 	int originalCodeStart = engine.interpreter.codeStart;
+
 	initEngineNode(engine.rootNode, &engine.interpreter, tb, size);
+
 	engine.interpreter.bytecode = originalTb;
 	engine.interpreter.length = originalSize;
 	engine.interpreter.count = originalCount;