Fix a bug where polymorphic fields could throw an exception (#1121)

lib/administrate/field/polymorphic.rb

@@ -30,7 +30,7 @@ def associated_dashboard(klass = data.class)
       end
 
       def classes
-        options.fetch(:classes) || []
+        options.fetch(:classes, [])
       end
 
       private

spec/lib/fields/polymorphic_spec.rb

@@ -51,4 +51,25 @@ def display_resource(*)
       expect(field.selected_global_id).to eq(nil)
     end
   end
+
+  describe "#classes" do
+    let(:field) { Administrate::Field::Polymorphic.new(:foo, "hello", :show) }
+
+    context "not present in options" do
+      it "returns an empty array" do
+        allow(field).to receive(:options).and_return({})
+
+        expect(field.send(:classes)).to eq([])
+      end
+    end
+
+    context "present in options" do
+      it "returns an present value" do
+        classes = ["one", "two", "three"]
+        allow(field).to receive(:options).and_return(classes: classes)
+
+        expect(field.send(:classes)).to eq(classes)
+      end
+    end
+  end
 end