Fixed possible ImGui assertions

[Lukas1811]

There were two cases were ImGui::EndChild was only called when BeginChild returned true.
This could lead to some assertions in ImGui like mentioned in #943.

Begin/BeginChild and End/EndChild are special cases according to comments in ImGui at the definition of Begin and End:

// - Begin() return false to indicate the window is collapsed or fully clipped, so you may early out and omit submitting
//   anything to the window. Always call a matching End() for each Begin() call, regardless of its return value!
//   [Important: due to legacy reason, Begin/End and BeginChild/EndChild are inconsistent with all other functions
//    such as BeginMenu/EndMenu, BeginPopup/EndPopup, etc. where the EndXXX call should only be called if the corresponding
//    BeginXXX function returned true. Begin and BeginChild are the only odd ones out. Will be fixed in a future update.]