Menu handler crashes on close window in release mode

[kmilo9999]

This problem is related to:
brown-ccv/VR-Volumeviewer#48

[kmilo9999]

It crashes on line imgui.cpp:2917

ImGuiWindow::~ImGuiWindow()
{
    IM_ASSERT(DrawList == &DrawListInst);
    IM_DELETE(Name);
    **for (int i = 0; i != ColumnsStorage.Size; i++)**
        ColumnsStorage[i].~ImGuiOldColumns();
}

the reference to ColumnsStorage goes to null and the code is not checking for null states at that point. ImGui 1.8.4 (we have 1.8.3) already fixes this issue changing the implementation of ColumnsStorage.

Solutions:

• Go up on Imgui >= 1.8.4.2