Fix calls to Fields::Types::Enum#enum_method when no bindings provided

Similar to `Fields::Types::Enum#enum` the code path using `abstract_model` was untested and the implementation did not account for the possibility that the `bindings` may be nil.

Author: robotfelix

lib/rails_admin/config/fields/types/enum.rb

@@ -21,7 +21,7 @@ class Enum < RailsAdmin::Config::Fields::Base
           end
 
           register_instance_option :enum_method do
-            @enum_method ||= bindings[:object].class.respond_to?("#{name}_enum") || (bindings[:object] || abstract_model.model.new).respond_to?("#{name}_enum") ? "#{name}_enum" : name
+            @enum_method ||= bindings&.[](:object).class.respond_to?("#{name}_enum") || (bindings&.[](:object) || abstract_model.model.new).respond_to?("#{name}_enum") ? "#{name}_enum" : name
           end
 
           register_instance_option :enum do

spec/rails_admin/config/fields/types/enum_spec.rb

@@ -17,11 +17,17 @@
       end
     end
 
-    it 'auto-detects enumeration' do
+    it 'auto-detects enumeration when bindings provide object' do
       is_expected.to be_a(RailsAdmin::Config::Fields::Types::Enum)
       is_expected.not_to be_multiple
       expect(subject.with(object: Team.new).enum).to eq %w[blue green red]
     end
+
+    it 'auto-detects enumeration when no bindings provided' do
+      is_expected.to be_a(RailsAdmin::Config::Fields::Types::Enum)
+      is_expected.not_to be_multiple
+      expect(subject.enum).to eq %w[blue green red]
+    end
   end
 
   describe "when class responds to '\#{method}_enum'" do
@@ -39,10 +45,15 @@ def color_enum
       end
     end
 
-    it 'auto-detects enumeration' do
+    it 'auto-detects enumeration when bindings provide object' do
       is_expected.to be_a(RailsAdmin::Config::Fields::Types::Enum)
       expect(subject.with(object: Team.new).enum).to eq %w[blue green red]
     end
+
+    it 'auto-detects enumeration when no bindings provided' do
+      is_expected.to be_a(RailsAdmin::Config::Fields::Types::Enum)
+      expect(subject.enum).to eq %w[blue green red]
+    end
   end
 
   describe 'the enum instance method' do