Fix crash (and show error message) when default font is nullptr (fix #5208)

Probably an edge-case for users that are creating new themes.

src/app/console.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
@@ -48,9 +48,12 @@ public:
 
     // When the main window is closed, we should close the console (in
     // other case the main message loop will continue running for the
-    // console too).
-    m_mainWindowClosedConn = App::instance()->mainWindow()->Close.connect(
-      [this] { closeWindow(nullptr); });
+    // console too). The main window can be nullptr if the console is
+    // used to show an error when loading the default theme or font at
+    // the initialization.
+    if (auto* mainWin = App::instance()->mainWindow()) {
+      m_mainWindowClosedConn = mainWin->Close.connect([this] { closeWindow(nullptr); });
+    }
 
     // When the window is closed, we clear the text
     Close.connect([this] {

src/app/ui/skin/skin_theme.cpp

@@ -545,6 +545,13 @@ void SkinTheme::loadXml(BackwardCompatibility* backward)
 
         text::FontRef font = fontData->getFont(m_fontMgr, size * ui::guiscale());
 
+        // No font?
+        if (font == nullptr) {
+          LOG(ERROR, "THEME: Error loading font for theme %s\n", idStr);
+          xmlFont = xmlFont->NextSiblingElement();
+          continue;
+        }
+
         // SpriteSheetFonts have a default preferred size.
         if (size == 0.0f && font->defaultSize() > 0.0f) {
           size = font->defaultSize();