diff --git a/src/__mocks__/@grafana/ui.tsx b/src/__mocks__/@grafana/ui.tsx
index a9e1276..c14b4d2 100644
--- a/src/__mocks__/@grafana/ui.tsx
+++ b/src/__mocks__/@grafana/ui.tsx
@@ -12,7 +12,8 @@ const Select = jest.fn(({ options, onChange, value, isMulti, isClearable, ...res
         if (isMulti) {
           onChange(options.filter((option: any) => event.target.values.includes(option.value)));
         } else {
-          onChange(options.find((option: any) => option.value === event.target.value));
+          const option = options.find((option: any) => option.value === event.target.value);
+          option && onChange(option);
         }
       }
     }}
diff --git a/src/components/DatasetEditor/DatasetEditor.test.tsx b/src/components/DatasetEditor/DatasetEditor.test.tsx
index f53ffbe..b65190d 100644
--- a/src/components/DatasetEditor/DatasetEditor.test.tsx
+++ b/src/components/DatasetEditor/DatasetEditor.test.tsx
@@ -159,6 +159,30 @@ describe('Dataset Editor', () => {
     expect(selectors.item(true, 'A:Value')).not.toBeInTheDocument();
   });
 
+  it('Should not allow selecting already selected fields', () => {
+    const { value, onChange } = createOnChangeHandler([{ name: 'Time', source: 'A' }]);
+
+    /**
+     * Render
+     */
+    render(
+      getComponent({
+        value,
+        onChange,
+      })
+    );
+
+    const newItemNameInput = selectors.newItemName();
+
+    /**
+     * Simulate select option doesn't exist
+     */
+
+    fireEvent.change(newItemNameInput, { target: { value: 'A:Time' } });
+
+    expect(selectors.buttonAddNew()).toBeDisabled();
+  });
+
   /**
    * Items order
    */