Docking, Modal: Fixed a crash when opening popup from a parent which is being docked on the same frame. (#5401)

Ideally we should untangle the purpose of parent_window_in_stack / ParentWindowInBeginStack better.

docs/CHANGELOG.txt

@@ -229,6 +229,7 @@ Docking+Viewports Branch:
   when io.ConfigDockingAlwaysTabBar is true. (#5324) [@rokups]
 - Docking: Fixed incorrect focus highlight on docking node when focusing empty central node
   or a child window which was manually injected into a dockspace window.
+- Docking, Modal: Fixed a crash when opening popup from a parent which is being docked on the same frame. (#5401)
 - Viewports: Fixed translating a host viewport from briefly altering the size of AlwaysAutoResize windows. (#5057)
 - Viewports: Fixed main viewport size not matching ImDrawData::DisplaySize for one frame during resize
   when multi-viewports are disabled. (#4900)

imgui.cpp

@@ -6464,7 +6464,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
     }
 
     // Parent window is latched only on the first call to Begin() of the frame, so further append-calls can be done from a different window stack
-    ImGuiWindow* parent_window_in_stack = window->DockIsActive ? window->DockNode->HostWindow : g.CurrentWindowStack.empty() ? NULL : g.CurrentWindowStack.back().Window;
+    ImGuiWindow* parent_window_in_stack = (window->DockIsActive && window->DockNode->HostWindow) ? window->DockNode->HostWindow : g.CurrentWindowStack.empty() ? NULL : g.CurrentWindowStack.back().Window;
     ImGuiWindow* parent_window = first_begin_of_the_frame ? ((flags & (ImGuiWindowFlags_ChildWindow | ImGuiWindowFlags_Popup)) ? parent_window_in_stack : NULL) : window->ParentWindow;
     IM_ASSERT(parent_window != NULL || !(flags & ImGuiWindowFlags_ChildWindow));