Update property drawers that support multiple objects, fix NRE

Author: gilzoide

Editor/CustomInspectors/ReadOnlyPropertyAttributeDrawer.cs

@@ -3,7 +3,7 @@
 
 namespace Gilzoide.ConditionalObjects.Editor
 {
-    [CustomPropertyDrawer(typeof(ReadOnlyPropertyAttribute))]
+    [CustomPropertyDrawer(typeof(ReadOnlyPropertyAttribute)), CanEditMultipleObjects]
     public class ReadOnlyPropertyAttributeDrawer : PropertyDrawer
     {
         public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)

Editor/CustomInspectors/SingleFieldPropertyDrawer.cs

@@ -3,7 +3,7 @@
 
 namespace Gilzoide.ConditionalObjects.Editor
 {
-    [CustomPropertyDrawer(typeof(ISingleFieldProperty), true)]
+    [CustomPropertyDrawer(typeof(ISingleFieldProperty), true), CanEditMultipleObjects]
     public class SingleFieldPropertyDrawer : PropertyDrawer
     {
         public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)

Runtime/Extensions/PresetExtensions.cs

@@ -19,6 +19,10 @@ public static void ExcludeAllProperties(this Preset preset)
         public static void GetIncludedProperties(this Preset preset, HashSet<string> set)
         {
             set.Clear();
+            if (preset.PropertyModifications == null)
+            {
+                return;
+            }
             set.UnionWith(preset.PropertyModifications.Select(prop => prop.propertyPath));
             set.UnionWith(preset.PropertyModifications.Select(prop => prop.propertyPath.Split('.', 2)[0]));
         }

Runtime/PropertyModifier.cs

@@ -6,7 +6,7 @@ public class PropertyModifier : AImportTimeObjectModifier
     {
 #if UNITY_EDITOR
         [Space]
-        [Tooltip("Properties that should be modified in the imported prefab/scene if the conditions above are met")]
+        [Tooltip("Target object that will be modified in the imported prefab/scene if the conditions above are met")]
         public GameObjectOrComponent Target;
 
         protected override void Apply(bool filtersMatch)