Fix crash applying filters (fix #4928)

The program was crashing when applying a filter that as a result
removed the current cel

src/app/commands/filters/filter_manager_impl.cpp

@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2019-2023  Igara Studio S.A.
+// Copyright (C) 2019-2025  Igara Studio S.A.
 // Copyright (C) 2001-2018  David Capello
 //
 // This program is distributed under the terms of
@@ -251,7 +251,8 @@ void FilterManagerImpl::apply()
 
   ASSERT(m_reader.context());
   m_reader.context()->setCommandResult(result);
-  init(m_site.cel());
+  if (m_site.cel())
+    init(m_site.cel());
 }
 
 void FilterManagerImpl::applyToTarget()

src/app/commands/filters/filter_manager_impl.h

@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2019-2023  Igara Studio S.A.
+// Copyright (C) 2019-2025  Igara Studio S.A.
 // Copyright (C) 2001-2018  David Capello
 //
 // This program is distributed under the terms of
@@ -100,6 +100,7 @@ public:
   doc::Sprite* sprite() { return m_site.sprite(); }
   doc::Layer* layer() { return m_site.layer(); }
   doc::frame_t frame() { return m_site.frame(); }
+  doc::Cel* cel() { return m_site.cel(); }
   doc::Image* destinationImage() const { return m_dst.get(); }
   gfx::Point position() const { return gfx::Point(0, 0); }
 

src/app/commands/filters/filter_window.cpp

@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2020-2023  Igara Studio S.A.
+// Copyright (C) 2020-2025  Igara Studio S.A.
 // Copyright (C) 2001-2018  David Capello
 //
 // This program is distributed under the terms of
@@ -166,6 +166,13 @@ void FilterWindow::onApply()
   update_screen_for_document(m_filterMgr->document());
 
   restartPreview();
+
+  // If there is no cel after applying the filter, then close the window because we cannot
+  // continue applying it over an empty cel.
+  if (!m_filterMgr->cel()) {
+    onCancel();
+    return;
+  }
 }
 
 void FilterWindow::onOk()

src/app/ui/timeline/timeline.cpp

@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2018-2024  Igara Studio S.A.
+// Copyright (C) 2018-2025  Igara Studio S.A.
 // Copyright (C) 2001-2018  David Capello
 //
 // This program is distributed under the terms of
@@ -1958,7 +1958,7 @@ void Timeline::onAddCel(DocEvent& ev)
 
 void Timeline::onAfterRemoveCel(DocEvent& ev)
 {
-  invalidateLayer(ev.layer());
+  ui::execute_now_or_enqueue([this, ev] { invalidateLayer(ev.layer()); });
 }
 
 void Timeline::onLayerNameChange(DocEvent& ev)

src/ui/system.cpp

@@ -1,5 +1,5 @@
 // Aseprite UI Library
-// Copyright (C) 2018-2024  Igara Studio S.A.
+// Copyright (C) 2018-2025  Igara Studio S.A.
 // Copyright (C) 2001-2018  David Capello
 //
 // This file is released under the terms of the MIT license.
@@ -280,6 +280,14 @@ void execute_from_ui_thread(std::function<void()>&& func)
   os::queue_event(ev);
 }
 
+void execute_now_or_enqueue(std::function<void()>&& func)
+{
+  if (is_ui_thread())
+    func();
+  else
+    execute_from_ui_thread(std::move(func));
+}
+
 bool is_ui_thread()
 {
   return (main_gui_thread == std::this_thread::get_id());

src/ui/system.h

@@ -1,5 +1,5 @@
 // Aseprite UI Library
-// Copyright (C) 2019-2024  Igara Studio S.A.
+// Copyright (C) 2019-2025  Igara Studio S.A.
 // Copyright (C) 2001-2018  David Capello
 //
 // This file is released under the terms of the MIT license.
@@ -62,6 +62,9 @@ gfx::Point get_mouse_position();
 void set_mouse_position(const gfx::Point& newPos, Display* display);
 
 void execute_from_ui_thread(std::function<void()>&& func);
+// If it is called from the UI thread just executes the function, if it is
+// called from a different thread, then call execute_from_ui_thread.
+void execute_now_or_enqueue(std::function<void()>&& func);
 bool is_ui_thread();
 #ifdef _DEBUG
 void assert_ui_thread();