Dashboards with a non element child fail to render or provide feedback

[mattrunyon]

Description

If a dashboard is not passed a ui.component instance as its child parameter, it will open a dashboard, but nothing will render. The most basic example is something like d = ui.dashboard(4). A more plausible example is d = ui.dashboard(my_component) where my_component should have been called to create the ui.component, but it isn't.

Steps to reproduce

import deephaven.ui as ui

d = ui.dashboard(4)

Expected results

Python error or at least an error printed to the dashboard that opens.

Actual results

No error. Dashboard opens and displays nothing.

[mofojed]

Related to #85
Also if you have a component that doesn't output anything (e.g. empty children), we should show some sort of message/toast to the user so they're not confused about nothing opening when they click to open it.