Fix plugin unloading

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## [Unreleased]
 * Improved server instance ordering behavior when files are added during a live session ([#135](https://github.com/LPGhatguy/rojo/pull/135))
+* Fixed error being thrown when trying to unload the Rojo plugin.
 
 ## [0.5.0 Alpha 6](https://github.com/LPGhatguy/rojo/releases/tag/v0.5.0-alpha.6) (March 19, 2019)
 * Fixed `rojo init` giving unexpected results by upgrading to `rbx_dom_weak` 1.1.0

plugin/src/init.server.lua

@@ -10,8 +10,8 @@ local app = Roact.createElement(App, {
 	plugin = plugin,
 })
 
-Roact.mount(app, game:GetService("CoreGui"), "Rojo UI")
+local tree = Roact.mount(app, game:GetService("CoreGui"), "Rojo UI")
 
 plugin.Unloading:Connect(function()
-	Roact.unmount(app)
+	Roact.unmount(tree)
 end)