Using EditorGUILayout in my PropertyDrawer for an attribute causes ArgumentException

I have an attribute for strings that does validation and draws a search field to lookup possible values. I wrote it back on 2018.3, but now I'm on 2019.4 and it doesn't work. In fact, nothing with EditorGUILayout seems to work. Even this simple example causes "ArgumentException: Getting control 1's position in a group with only 1 controls when doing repaint":

 // Editor/TestStringAttribute_Editor.cs
 using System.Collections.Generic;
 using System.Collections;
 using System.Linq;
 using System;
 using UnityEditor;
 using UnityEngine;
 
 namespace TestCode {
 
     // Editor for attribute that provides autocompletion buttons
     //      [TestString] public string m_Key;
     [CustomPropertyDrawer(typeof(TestStringAttribute))]
     public class TestStringDrawer : PropertyDrawer {
         public override float GetPropertyHeight(SerializedProperty property, GUIContent label) {
             // Add a bit more space for text when narrow.
             return EditorGUIUtility.singleLineHeight * (EditorGUIUtility.wideMode ? 1f : 2f);
         }
 
         public override void OnGUI(Rect position, SerializedProperty property, GUIContent label) {        
             var key = property.stringValue;
             EditorGUI.BeginChangeCheck();
             // This causes "ArgumentException: Getting control 1's position in a group with only 1 controls when doing repaint"
             var was_visible = Ehttps://docs.unity3d.com/ScriptReference/PropertyDrawer.htmlditorGUILayout.PropertyField(property, label);
             // This works, but I don't want to calculate the position for all my other controls!
             //~ var was_visible = EditorGUI.PropertyField(position, property, label);
             if (EditorGUI.EndChangeCheck()) {
                 property.stringValue = key;
             }
         }
 
     }
 }
 
 // TestStringAttribute.cs
 using UnityEngine;
 
 namespace TestCode {
     // Defines an attribute that provides autocompletion buttons
     // See TestStringAttribute_Editor.cs for implementation.
     // Use like this:
     //      [TestString] public string m_Key;
     public class TestStringAttribute : PropertyAttribute {}
 }
 
 // TestBehaviour.cs
 using System.Collections.Generic;
 using System.Collections;
 using System.Linq;
 using System;
 using UnityEngine;
 
 using Object = UnityEngine.Object;
 
 namespace TestCode {
     public class TestBehaviour : MonoBehaviour {
         [TestString] public string m_Key;
     }
 }

How do I make this work?