eeschema: fix crash when closing simulation frame

Destructor of SIM_PLOT_FRAME calls sim->Attach( nullptr ) in order to destroy
circuit model (former netlist exporter). If this is skipped, eeschema crashes
when program is closed. nullptr is a valid parameter for Attach()

eeschema/sim/ngspice.cpp

@@ -258,10 +258,9 @@ vector<double> NGSPICE::GetPhasePlot( const string& aName, int aMaxLen )
 bool NGSPICE::Attach( const std::shared_ptr<SIMULATION_MODEL>& aModel )
 {
     NGSPICE_CIRCUIT_MODEL* model = dynamic_cast<NGSPICE_CIRCUIT_MODEL*>( aModel.get() );
-    wxASSERT( model != nullptr );
     STRING_FORMATTER formatter;
 
-    if( model->GetNetlist( &formatter ) )
+    if( model && model->GetNetlist( &formatter ) )
     {
         SIMULATOR::Attach( aModel );