It seem GTK has had similar problems on Mac before, seemingly related to how the (non-julia) GTK library is installed on the system. Until more knowledgeable people show up, this search might be a good start julia gtk on mac blank window - Google Search
But if you do not want to bother debugging this, I would suggest just using the SVG backend for flamegraphs https://github.com/kimikage/ProfileSVG.jl