How to reproduce:

• Open the Skin factory tool with user_1 in browser_1
• Modify something in the skin
• Open the Skin factory tool with user_2 in browser_2
• The dialog box informs you that somebody is already working on the sin => OK
• Int his dialog box, the message specifies the tool with wich is working user_1, but this tool is 'undefined' instead of Skin factory => KO (see capture)