Prevent command line applications handling dark mode

Double-check before getting background color from invalid m_parent also

Fixes https://gitlab.com/kicad/code/kicad/-/issues/21317

(cherry picked from commit 9cb3a37b6f)

common/widgets/grid_color_swatch_helpers.cpp

@@ -116,7 +116,7 @@ void GRID_CELL_COLOR_RENDERER::Draw( wxGrid& aGrid, wxGridCellAttr& aAttr, wxDC&
 
 void GRID_CELL_COLOR_RENDERER::OnDarkModeToggle()
 {
-    m_checkerboardBg = m_parent->GetBackgroundColour();
+    m_checkerboardBg = m_parent ? m_parent->GetBackgroundColour() : wxSystemSettings::GetColour( wxSYS_COLOUR_WINDOW );
 }
 
 

pcbnew/pcb_base_edit_frame.cpp

@@ -60,6 +60,10 @@ PCB_BASE_EDIT_FRAME::PCB_BASE_EDIT_FRAME( KIWAY* aKiway, wxWindow* aParent,
 {
     m_darkMode = KIPLATFORM::UI::IsDarkTheme();
 
+    // Do not register the idle event handler if we are running in headless mode.
+    if( !wxApp::GetGUIInstance() )
+        return;
+
     Bind( wxEVT_IDLE,
           [this]( wxIdleEvent& aEvent )
           {