Added assert in TreePop() to ease catching ID stack underflow earlier.

imgui.cpp

@@ -13323,6 +13323,7 @@ void ImGui::TreePop()
         }
     window->DC.TreeDepthMayJumpToParentOnPop &= (1 << window->DC.TreeDepth) - 1;
 
+    IM_ASSERT(window->IDStack.Size > 1); // There should always be 1 element in the IDStack (pushed during window creation). If this triggers you called TreePop/PopID too much.
     PopID();
 }