Resource TMDIChild not found

Question

I am writing an MDI application. It works fine in Delphi 10.3 + Windows 10, but I get the error "Resource TMDIChild not found" in Delphi 10.4 + Windows 10. What is the problem, and how to solve it? Thanks.

  if frmMDIChild = nil then
    frmMDIChild := TMDIChild.Create(Application); { errors occur here!!! }
  frmMDIChildForm.Show;