Don't crash when loading bitmaps before settings exist

Fixes https://gitlab.com/kicad/code/kicad/-/issues/7900

common/bitmap_store.cpp

@@ -170,7 +170,7 @@ bool BITMAP_STORE::ThemeChanged()
 
     wxString oldTheme = m_theme;
 
-    if( ADVANCED_CFG::GetCfg().m_AllowDarkMode )
+    if( ADVANCED_CFG::GetCfg().m_AllowDarkMode && settings )
     {
         switch( settings->m_Appearance.icon_theme )
         {

common/pgm_base.cpp

@@ -583,7 +583,7 @@ void PGM_BASE::SaveCommonSettings()
 
 COMMON_SETTINGS* PGM_BASE::GetCommonSettings() const
 {
-    return GetSettingsManager().GetCommonSettings();
+    return m_settings_manager ? GetSettingsManager().GetCommonSettings() : nullptr;
 }