fix null object error on cancel export

tools/unity-avatar-exporter/Assets/Editor/AvatarExporter/AvatarExporter.cs

@@ -1531,6 +1531,7 @@ class ExportProjectWindow : EditorWindow {
     
     float GetAvatarHeight() {
         // height of an avatar model can be determined to be the max Y extents of the combined bounds for all its mesh renderers
+        if (avatarPreviewObject != null) {
             Bounds bounds = new Bounds();
             var meshRenderers = avatarPreviewObject.GetComponentsInChildren<MeshRenderer>();
             var skinnedMeshRenderers = avatarPreviewObject.GetComponentsInChildren<SkinnedMeshRenderer>();
@@ -1542,6 +1543,8 @@ class ExportProjectWindow : EditorWindow {
             }
             return bounds.max.y;
         }
+        return 0.0f;
+    }
     
     void SetAvatarScale(float actualScale) {
         // set the new scale uniformly on the preview avatar's transform to show the resulting avatar size