Eeschema: fix crash when exporting a netlist, if the schematic netlist cannot be built.

It happens when a symbol is not annotated, and if the user closes the opened annotate dialog without making the annotation

eeschema/dialogs/dialog_netlist.cpp

@@ -599,8 +599,11 @@ void NETLIST_DIALOG::GenNetlist( wxCommandEvent& event )
 
     auto netlist = m_Parent->CreateNetlist( false, false );
 
-    m_Parent->WriteNetListFile( netlist, currPage->m_IdNetType,
-                                fullpath, netlist_opt, NULL );
+    if( netlist == nullptr )
+        wxMessageBox( _( "Schematic netlist not available" ) );
+    else
+        m_Parent->WriteNetListFile( netlist, currPage->m_IdNetType,
+                                    fullpath, netlist_opt, NULL );
 
     WriteCurrentNetlistSetup();
 

eeschema/netlist_generator.cpp

@@ -51,6 +51,9 @@ bool SCH_EDIT_FRAME::WriteNetListFile( NETLIST_OBJECT_LIST* aConnectedItemsList,
                                        int aFormat, const wxString& aFullFileName,
                                        unsigned aNetlistOptions, REPORTER* aReporter )
 {
+    if( aConnectedItemsList == nullptr )    // Schematic netlist not available.
+        return false;
+
     bool res = true;
     bool executeCommandLine = false;