fix(variables): clear prompt_func after editing variable (#330)

Closes #325

---

`prompt_func` is not being reset to `nil` when editing a variable. This
causes `canvas:set_prompt()` to get trigger on every other action, like `expand`.

lua/dapui/components/variables.lua

@@ -82,9 +82,9 @@ return function(client, send_ready)
         canvas:add_mapping("edit", function()
           prompt_func = function(new_value)
             async.run(function()
-              client.lib.set_variable(parent_ref, variable, new_value)
               prompt_func = nil
               prompt_fill = nil
+              client.lib.set_variable(parent_ref, variable, new_value)
               send_ready()
             end)
           end