problem method setCacheMode in QGraphicsProxyWidget QT5

Hello all, I deleted a call to the function setCacheMode(QGraphicsItem::ItemCoordinateCache) on a QGraphicsProxyWidget because i want to invoke manually the method update() which calls the paint event of my QGraphicsItem (if a cache is defined, paint is never called sometimes ). But, i’ve got some problems now in order to show QGraphicsScene : the QGraphicsItems in the scene are never showed now. If I call the function setCacheMode(QGraphicsItem::ItemCoordinateCache) it’s OK. Any hint? Thanks