fix backend(r::AbstractREPL) fir REPLs with no backend

Author: IanButterworth

stdlib/REPL/src/REPL.jl

@@ -1145,7 +1145,7 @@ find_hist_file() = get(ENV, "JULIA_HISTORY",
                        !isempty(DEPOT_PATH) ? joinpath(DEPOT_PATH[1], "logs", "repl_history.jl") :
                        error("DEPOT_PATH is empty and ENV[\"JULIA_HISTORY\"] not set."))
 
-backend(r::AbstractREPL) = r.backendref
+backend(r::AbstractREPL) = hasproperty(r, :backendref) ? r.backendref : nothing
 
 
 function eval_with_backend(ast::Expr, backend::REPLBackendRef)