FIX: Return the right response code for invalid theme id.

app/controllers/admin/themes_controller.rb

@@ -7,8 +7,10 @@ class Admin::ThemesController < Admin::AdminController
   skip_before_action :check_xhr, only: [:show, :preview, :export]
 
   def preview
-    @theme = Theme.find(params[:id])
+    theme = Theme.find_by(id: params[:id])
-    redirect_to path("/?preview_theme_id=#{@theme.id}")
+    raise Discourse::InvalidParameters.new(:id) unless theme
+
+    redirect_to path("/?preview_theme_id=#{theme.id}")
   end
 
   def upload_asset
@@ -148,7 +150,8 @@ class Admin::ThemesController < Admin::AdminController
   end
 
   def update
-    @theme = Theme.find(params[:id])
+    @theme = Theme.find_by(id: params[:id])
+    raise Discourse::InvalidParameters.new(:id) unless @theme
 
     original_json = ThemeSerializer.new(@theme, root: false).to_json
 
@@ -215,7 +218,9 @@ class Admin::ThemesController < Admin::AdminController
   end
 
   def destroy
-    @theme = Theme.find(params[:id])
+    @theme = Theme.find_by(id: params[:id])
+    raise Discourse::InvalidParameters.new(:id) unless @theme
+
     StaffActionLogger.new(current_user).log_theme_destroy(@theme)
     @theme.destroy
 
@@ -225,12 +230,15 @@ class Admin::ThemesController < Admin::AdminController
   end
 
   def show
-    @theme = Theme.find(params[:id])
+    @theme = Theme.find_by(id: params[:id])
+    raise Discourse::InvalidParameters.new(:id) unless @theme
+
     render json: ThemeSerializer.new(@theme)
   end
 
   def export
-    @theme = Theme.find(params[:id])
+    @theme = Theme.find_by(id: params[:id])
+    raise Discourse::InvalidParameters.new(:id) unless @theme
 
     exporter = ThemeStore::TgzExporter.new(@theme)
     file_path = exporter.package_filename

spec/requests/admin/themes_controller_spec.rb

@@ -218,6 +218,12 @@ describe Admin::ThemesController do
   describe '#update' do
     let(:theme) { Fabricate(:theme) }
 
+    it 'returns the right response when an invalid id is given' do
+      put "/admin/themes/99999.json"
+
+      expect(response.status).to eq(400)
+    end
+
     it 'can change default theme' do
       SiteSetting.default_theme_id = -1
 
@@ -342,6 +348,12 @@ describe Admin::ThemesController do
   describe '#destroy' do
     let(:theme) { Fabricate(:theme) }
 
+    it 'returns the right response when an invalid id is given' do
+      delete "/admin/themes/9999.json"
+
+      expect(response.status).to eq(400)
+    end
+
     it "deletes the field's javascript cache" do
       theme.set_field(target: :common, name: :header, value: '<script>console.log("test")</script>')
       theme.save!
@@ -356,4 +368,12 @@ describe Admin::ThemesController do
       expect { javascript_cache.reload }.to raise_error(ActiveRecord::RecordNotFound)
     end
   end
+
+  describe '#preview' do
+    it "should return the right response when an invalid id is given" do
+      get "/admin/themes/9999/preview.json"
+
+      expect(response.status).to eq(400)
+    end
+  end
 end