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
• Click on Ok button to take the lead on skin edition
• Modify something in the skin
• Close the skin factory tool on browser_2
• With user_1 on browser_1, try to close the skin factory tool
• An error occured, there is an NPE ad you can't close the tool => KO (see capture and log file)