Fixed issue with de-selecting dropdown option in issue sidebar

Closes #18641

app/assets/javascripts/milestone_select.js.coffee

@@ -116,7 +116,7 @@ class @MilestoneSelect
               .val()
             data = {}
             data[abilityName] = {}
-            data[abilityName].milestone_id = selected
+            data[abilityName].milestone_id = if selected? then selected else null
             $loading
               .fadeIn()
             $dropdown.trigger('loading.gl.dropdown')

app/assets/javascripts/users_select.js.coffee

@@ -31,7 +31,7 @@ class @UsersSelect
       assignTo = (selected) ->
         data = {}
         data[abilityName] = {}
-        data[abilityName].assignee_id = selected
+        data[abilityName].assignee_id = if selected? then selected else null
         $loading
           .fadeIn()
         $dropdown.trigger('loading.gl.dropdown')

spec/features/issues_spec.rb

@@ -396,6 +396,27 @@ describe 'Issues', feature: true do
           expect(page).to have_content @user.name
         end
       end
+
+      it 'allows user to unselect themselves', js: true do
+        issue2 = create(:issue, project: project, author: @user)
+        visit namespace_project_issue_path(project.namespace, project, issue2)
+
+        page.within '.assignee' do
+          click_link 'Edit'
+          click_link @user.name
+
+          page.within '.value' do
+            expect(page).to have_content @user.name
+          end
+
+          click_link 'Edit'
+          click_link @user.name
+
+          page.within '.value' do
+            expect(page).to have_content "No assignee"
+          end
+        end
+      end
     end
 
     context 'by unauthorized user' do
@@ -440,6 +461,26 @@ describe 'Issues', feature: true do
 
         expect(issue.reload.milestone).to be_nil
       end
+
+      it 'allows user to de-select milestone', js: true do
+        visit namespace_project_issue_path(project.namespace, project, issue)
+
+        page.within('.milestone') do
+          click_link 'Edit'
+          click_link milestone.title
+
+          page.within '.value' do
+            expect(page).to have_content milestone.title
+          end
+
+          click_link 'Edit'
+          click_link milestone.title
+
+          page.within '.value' do
+            expect(page).to have_content 'None'
+          end
+        end
+      end
     end
 
     context 'by unauthorized user' do