FIX: Prevent users from converting the default theme to a component

This results in some fun disasters if allowed to happen. For now, just issue an oblique error message; a localized message will be added on the client.

app/controllers/admin/themes_controller.rb

@@ -401,8 +401,10 @@ class Admin::ThemesController < Admin::AdminController
   def handle_switch
     param = theme_params[:component]
     if param.to_s == "false" && @theme.component?
+      raise Discourse::InvalidParameters.new(:component) if @theme.id == SiteSetting.default_theme_id
       @theme.switch_to_theme!
     elsif param.to_s == "true" && !@theme.component?
+      raise Discourse::InvalidParameters.new(:component) if @theme.id == SiteSetting.default_theme_id
       @theme.switch_to_component!
     end
   end

spec/requests/admin/themes_controller_spec.rb

@@ -551,6 +551,18 @@ describe Admin::ThemesController do
       expect(response.status).to eq(400)
       expect(response.parsed_body["errors"].first).to include(I18n.t("themes.errors.component_no_default"))
     end
+
+    it 'prevents converting the default theme to a component' do
+      SiteSetting.default_theme_id = theme.id
+
+      put "/admin/themes/#{theme.id}.json", params: {
+        theme: { component: true }
+      }
+
+      # should this error message be localized? InvalidParameters :component
+      expect(response.status).to eq(400)
+      expect(response.parsed_body["errors"].first).to include('component')
+    end
   end
 
   describe '#destroy' do