Clean g.WindowsById storage on shutdown.
diff --git a/imgui.cpp b/imgui.cpp
index dc501d4..2a5195f 100644
--- a/imgui.cpp
+++ b/imgui.cpp
@@ -2462,6 +2462,7 @@
     g.WindowsSortBuffer.clear();
     g.CurrentWindow = NULL;
     g.CurrentWindowStack.clear();
+    g.WindowsById.Clear();
     g.NavWindow = NULL;
     g.HoveredWindow = NULL;
     g.HoveredRootWindow = NULL;