try this:
hi_window = guiCreateWindow(354,252,362,272,"Haushaltsinventar",false)
guiWindowSetSizable(hi_window,false)
--Label
hi_lbl = guiCreateLabel(34,25,305,35,"Hallo. Das ist dein Haushaltsinventar.\nVon hier aus kannst du Möbel in dein Haus platzieren.",false,hi_window)
guiSetFont(hi_lbl,"default-bold-small")
--Grid
hi_grid = guiCreateGridList(38,69,290,157,false,hi_window)
guiGridListSetSelectionMode(hi_grid,2)
hinvName = guiGridListAddColumn(hi_grid,"Name",2)
--Button
hi_btn = guiCreateButton(90,231,186,22,"Platzieren",false,hi_window)
centerWindow(hi_window)
addEventHandler("onClientGUIClick",root,function(button)
if(button=="left")then
if(source==hi_btn)then
guiSetVisible(hi_window,false)
textITT()
end
end
end,false)
I know what was wrong