Use of theme Iconless raises error when trying to expand top-level entry in errors view

[swinkels]

If I set the lsp-treemacs-theme to new theme Iconless, pressing the tab key on the top-level entry of the LSP Errors View results in a Wrong type argument: char-or-string-p, nil and the entry is not expanded. The stack trace shows the following:

Debugger entered--Lisp error: (wrong-type-argument char-or-string-p nil)
  treemacs--do-expand-lsp-projects(#<marker (moves after insertion) at 13 in *LSP Error List*>)
  treemacs-expand-lsp-projects(nil)
  treemacs-TAB-action(nil)
  funcall-interactively(treemacs-TAB-action nil)
  call-interactively(treemacs-TAB-action nil nil)
  command-execute(treemacs-TAB-action)

If I set the theme back to Default again (via Erase Customization), kill the LSP Errors View and display the view again, I get the same error. If I then restart Emacs, still using theme Default, display the LSP Errors View again and press the tab key on the top-level entry, no error is raised and the entry is expanded.

I tested this in Spacemacs develop branch, commit c4ad14a, dated Sunday December 20 2020, using a Python file and the pyls backend.