Don't raise exception when no model in bootstrap_form_with

lib/bootstrap_form/form_group_builder.rb

@@ -107,7 +107,7 @@ def form_group_css_options(method, html_options, options)
     end
 
     def form_group_placeholder(options, method)
-      form_group_label_text(options[:label]) || object.class.human_attribute_name(method)
+      form_group_label_text(options[:label]) || (object && object.class.human_attribute_name(method)) || method.to_s.humanize # rubocop:disable Style/SafeNavigation
     end
   end
 end

test/bootstrap_form_group_test.rb

@@ -118,6 +118,16 @@ class BootstrapFormGroupTest < ActionView::TestCase
     assert_equivalent_html expected, @builder.text_field(:email, label_as_placeholder: true)
   end
 
+  test "label as placeholder with form_with" do
+    expected = <<~HTML
+      <div class="mb-3">
+        <label class="visually-hidden required" for="user_email">Email</label>
+        <input required="required" class="form-control" id="user_email" placeholder="Email" name="user[email]" type="text" value="steve@example.com" />
+      </div>
+    HTML
+    assert_equivalent_html expected, form_with_builder.text_field(:email, label_as_placeholder: true, required: true)
+  end
+
   test "adding prepend text" do
     expected = <<~HTML
       <div class="mb-3">

test/test_helper.rb

@@ -41,10 +41,10 @@ def setup_test_fixture
   end
 
   # Originally only used in one test file but placed here in case it's needed in others in the future.
+  # This simulartes `form_with` without a model, which tests aspects that wouldn't otherwise
+  # be tested.
   def form_with_builder
-    builder = nil
-    bootstrap_form_with(model: @user) { |f| builder = f }
-    builder
+    BootstrapForm::FormBuilder.new(:user, false, self, {})
   end
 
   def sort_attributes(doc)