lua: Dialog() must return nil if there is no UI available

This avoids some crashes when running in --batch mode and some script
tries to create a Dialog().

src/app/commands/cmd_run_script.cpp

@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2018  Igara Studio S.A.
+// Copyright (C) 2018-2022  Igara Studio S.A.
 // Copyright (C) 2001-2018  David Capello
 //
 // This program is distributed under the terms of
@@ -80,7 +80,10 @@ void RunScriptCommand::onExecute(Context* context)
     ->scriptEngine()
     ->evalFile(m_filename, m_params);
 
-  ui::Manager::getDefault()->invalidate();
+#if ENABLE_UI
+  if (context->isUIAvailable())
+    ui::Manager::getDefault()->invalidate();
+#endif
 }
 
 std::string RunScriptCommand::onGetFriendlyName() const

src/app/script/api_version.h

@@ -10,6 +10,6 @@
 
 // Increment this value if the scripting API is modified between two
 // released Aseprite versions.
-#define API_VERSION   17
+#define API_VERSION   18
 
 #endif

src/app/script/dialog_class.cpp

@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2018-2020  Igara Studio S.A.
+// Copyright (C) 2018-2022  Igara Studio S.A.
 // Copyright (C) 2018  David Capello
 //
 // This program is distributed under the terms of
@@ -201,6 +201,10 @@ void Dialog_connect_signal(lua_State* L,
 
 int Dialog_new(lua_State* L)
 {
+  // If we don't have UI, just return nil
+  if (!App::instance()->isGui())
+    return 0;
+
   auto dlg = push_new<Dialog>(L);
 
   // The uservalue of the dialog userdata will contain a table that