Received Memory Warning then crash iOS but I release resources?

I load into a VR photosphere Google 360 Panorama and my own Richoh Theta s 360 photos and all goes well.

In total my menu has 16 choices and when I choose to load number 10 I get an iOS memory warning but the app displays the photo and still runs.

When I go to load number 11 I get another warning then crash.

In my c# code that changes the photosphere material I "believe" I unload each previous material so presume I should be ok?

     if (currentMaterial != null) {
                 
                 Resources.UnloadAsset (currentMaterial);
             }
 
             // otherwise process for photo
             currentMaterial = Resources.Load(materialName, typeof(Material)) as Material;
 
             photoSphere.GetComponent<Renderer>().material = currentMaterial;

The thing is...I don't know how or what to enter in the iOS didReceiveMemery warning function? I know that is the place to do some damage limitation but I don't know how or what to reference?