By default, the built-in Find All widget uses the “Find All” shortcut for it’s PluginAction. This default PluginAction shortcut does not allow you to hide the widget - only make it visible.
If the default shortcut was changed to the “Find All / Replace All” PluginAction, it would solve this issue and let developers press the same keyboard shortcut twice in a row to both be able to show and hide the widget at a whim.
Right now, this change has to be done manually by the developer; something I imagine not many know exist because of the current way that Roblox Studio keyboard shortcuts are surfaced.