destroy instances and release resources

[Daisiyou]

Describe the bug
when I call method "setMaterialInstanceAt", How should I destroy old material instances?

MyCode: (iOS)

 RenderableManager& rcm = _engine->getRenderableManager();
 MaterialInstance* matIns = rcm.getMaterialInstanceAt(rcm.getInstance(entity), 0);
    if (matIns != NULL) {
        NSLog(@"find partName :%@", partName);
        NSData *data = [NSData dataWithContentsOfFile:materialPath];
        size_t size = data.length;
        Material* mat = Material::Builder()
            .package([data bytes], size)
            .build(*_engine);
        MaterialInstance* mInstance = mat->createInstance();
        rcm.setMaterialInstanceAt(rcm.getInstance(entity), 0, mInstance);
        
        //release resource
        _engine->destroy(matIns->getMaterial());
        _engine->destroy(matIns);
  }

Logs

in destroy:994
reason: destroying material "volume_ubershader" but 10 instances still alive
#0 FFChat 0x106844cc0 _ZN5utils7details8panicLogEPKcS2_iS2_z + 260
#1 FFChat 0x1067d0d78 _ZN8filament7FEngine7destroyEPKNS_9FMaterialE + 208
#2 FFChat 0x106e2a720 -[FF_3DApp changeModelPart:withPartName:andMaterial:] + 864
#3 FFChat 0x1044a4ca8 $s6FFChat10FFUser3DVCC10beardClickyySo8UIButtonCF + 620
#4 FFChat 0x1044a5544 $s6FFChat10FFUser3DVCC10beardClickyySo8UIButtonCFTo + 52
#5 UIKitCore 0x18ee0b908 + 100
#6 UIKitCore 0x18ee0b608 + 112
#7 UIKitCore 0x18ee0af9c + 324
#8 UIKitCore 0x18ee0ae40 + 124
#9 UIKitCore 0x18ee0aa14 + 400
#10 FFChat 0x106e25260 -[UIButton(BUATouchTracker) touchesEnded:withEvent:] + 1208
#11 UIKitCore 0x18ecf0268 + 5772
#12 CoreFoundation 0x18ca79d40 + 36
#13 CoreFoundation 0x18ca02290 + 532
#14 CoreFoundation 0x18ca63c60 + 1048
#15 CoreFoundation 0x18ca68ec0 CFRunLoopRunSpecific + 612
#16 GraphicsServices 0x1c6abf368 GSEventRunModal + 164
#17 UIKitCore 0x18ef5e86c + 888

[romainguy]

You can't destroy a material if you haven't destroyed its instances still in use.