Fix crash when trying to access a property of a Style which is nullptr (

fix aseprite#4015)

Before this fix, an incomplete custom theme or an outdated official
theme could cause a crash during Aseprite startup.

This fix does not alert the artist the problem of the theme.
Simply avoid the crash.

src/app/ui/skin/skin_theme.h

@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2020-2023  Igara Studio S.A.
+// Copyright (C) 2020-2024  Igara Studio S.A.
 // Copyright (C) 2001-2017  David Capello
 //
 // This program is distributed under the terms of
@@ -108,7 +108,7 @@ namespace app {
         if (it != m_styles.end())
           return it->second;
         else
-          return nullptr;
+          return getDefaultStyle();
       }
 
       SkinPartPtr getPartById(const std::string& id) const {

src/ui/grid.cpp

@@ -1,5 +1,5 @@
 // Aseprite UI Library
-// Copyright (C) 2018-2023  Igara Studio S.A.
+// Copyright (C) 2018-2024  Igara Studio S.A.
 // Copyright (C) 2001-2017  David Capello
 //
 // This file is released under the terms of the MIT license.
@@ -125,6 +125,9 @@ Grid::Info Grid::getChildInfo(Widget* child)
 
 void Grid::setStyle(Style* style)
 {
+  ASSERT(style);
+  if (!style)
+    style = Theme::getDefaultStyle();
   Widget::setStyle(style);
   setGap(style->gap());
 }

src/ui/theme.cpp

@@ -1,5 +1,5 @@
 // Aseprite UI Library
-// Copyright (C) 2019-2023  Igara Studio S.A.
+// Copyright (C) 2019-2024  Igara Studio S.A.
 // Copyright (C) 2001-2018  David Capello
 //
 // This file is released under the terms of the MIT license.
@@ -142,6 +142,9 @@ Theme::~Theme()
     set_theme(nullptr, guiscale());
 }
 
+// static
+std::unique_ptr<ui::Style> Theme::m_defaultStyle;
+
 void Theme::regenerateTheme()
 {
   set_mouse_cursor(kNoCursor);

src/ui/theme.h

@@ -1,5 +1,5 @@
 // Aseprite UI Library
-// Copyright (C) 2020-2022  Igara Studio S.A.
+// Copyright (C) 2020-2024  Igara Studio S.A.
 // Copyright (C) 2001-2017  David Capello
 //
 // This file is released under the terms of the MIT license.
@@ -141,6 +141,14 @@ namespace ui {
     static void drawTextBox(Graphics* g, const Widget* textbox,
                             int* w, int* h, gfx::Color bg, gfx::Color fg);
 
+    static ui::Style* getDefaultStyle() {
+      if (!m_defaultStyle.get()) {
+        m_defaultStyle.reset();
+        m_defaultStyle = std::make_unique<ui::Style>(nullptr);
+      }
+      return m_defaultStyle.get();
+    };
+
   protected:
     virtual void onRegenerateTheme() = 0;
 
@@ -165,6 +173,7 @@ namespace ui {
                            gfx::Size& sizeHint,
                            gfx::Border& borderHint,
                            gfx::Rect& textHint, int& textAlign);
+    static std::unique_ptr<ui::Style> m_defaultStyle;
   };
 
 } // namespace ui

src/ui/widget.cpp

@@ -1,5 +1,5 @@
 // Aseprite UI Library
-// Copyright (C) 2018-2023  Igara Studio S.A.
+// Copyright (C) 2018-2024  Igara Studio S.A.
 // Copyright (C) 2001-2018  David Capello
 //
 // This file is released under the terms of the MIT license.
@@ -199,7 +199,9 @@ void Widget::setTheme(Theme* theme)
 void Widget::setStyle(Style* style)
 {
   assert_ui_thread();
-
+  ASSERT(style);
+  if (!style)
+    style = Theme::getDefaultStyle();
   m_style = style;
   m_border = m_theme->calcBorder(this, style);
   m_bgColor = m_theme->calcBgColor(this, style);