.Net - ToolControl closing form

I'm developping some custom tools in Montaj .Net and I'm using ToolControl form for my user interaction. When I'm opening my Montaj project, I see multiple ToolControl window flashing like the software open each form I opened in my previous session and it's making the loading longer by each opening of the same project. When I'm opening a newly created project, I don't have these flashing windows. I think it's because I'm not closing my forms when I'm done with them but I didn't found any control to close the form. Does somebody encountered this problem and found a solution?