Disable non-KiCad import for project-mode

Adds warning if project schematic contains data

Fixes https://gitlab.com/kicad/code/kicad/-/issues/15570

eeschema/files-io.cpp

@@ -618,6 +618,15 @@ void SCH_EDIT_FRAME::OnImportProject( wxCommandEvent& aEvent )
     if( !AskToSaveChanges() )
         return;
 
+    if( Schematic().RootScreen() && !Schematic().RootScreen()->Items().empty() )
+    {
+        wxString msg = _( "This operation replaces the contents of the current project.\n\n"
+                          "Do you want to proceed?" );
+
+        if( !IsOK( this, msg ) )
+            return;
+    }
+
     // Set the project location if none is set or if we are running in standalone mode
     bool     setProject = Prj().GetProjectFullName().IsEmpty() || Kiface().IsSingle();
     wxString path = wxPathOnly( Prj().GetProjectFullName() );

eeschema/menubar.cpp

@@ -106,11 +106,15 @@ void SCH_EDIT_FRAME::doReCreateMenuBar()
     ACTION_MENU* submenuImport = new ACTION_MENU( false, selTool );
     submenuImport->SetTitle( _( "Import" ) );
     submenuImport->SetIcon( BITMAPS::import );
-    submenuImport->Add( _( "Non-KiCad Schematic..." ),
+
+    if( Kiface().IsSingle() )
+    {
+        submenuImport->Add( _( "Non-KiCad Schematic..." ),
                         _( "Replace current schematic sheet with one imported from another "
                            "application" ),
                         ID_IMPORT_NON_KICAD_SCH,
                         BITMAPS::import_document );
+    }
 
     submenuImport->Add( EE_ACTIONS::importFPAssignments, ACTION_MENU::NORMAL,
                         _( "Footprint Assignments..." ) );