Make sure nested settings get released in dtor

Also fix a crash if sim window can't be created because
ngspice shared library could not be found

Fixes https://gitlab.com/kicad/code/kicad/-/issues/7969

eeschema/schematic_settings.cpp

@@ -217,6 +217,9 @@ SCHEMATIC_SETTINGS::SCHEMATIC_SETTINGS( JSON_SETTINGS* aParent, const std::strin
 
 SCHEMATIC_SETTINGS::~SCHEMATIC_SETTINGS()
 {
+    ReleaseNestedSettings( m_NgspiceSimulatorSettings.get() );
+    m_NgspiceSimulatorSettings.reset();
+
     if( m_parent )
     {
         m_parent->ReleaseNestedSettings( this );

eeschema/tools/ee_inspection_tool.cpp

@@ -442,6 +442,10 @@ int EE_INSPECTION_TOOL::RunSimulation( const TOOL_EVENT& aEvent )
 {
 #ifdef KICAD_SPICE
     SIM_PLOT_FRAME* simFrame = (SIM_PLOT_FRAME*) m_frame->Kiway().Player( FRAME_SIMULATOR, true );
+
+    if( !simFrame )
+        return -1;
+
     simFrame->Show( true );
 
     // On Windows, Raise() does not bring the window on screen, when iconized