Hi,
I had the same error using panel grid with infine scrolling in Ext 4.1: changing pages with different grids, Ext always returned this error.
I solved it overriding two methods in Ext.grid.PagingScroller, this is the code:

Code:
Ext.override(Ext.grid.PagingScroller, {
    getFirstVisibleRowIndex: function() {
        var me = this,
            store = me.store,
            view = me.view,
            scrollTop = view.el.dom.scrollTop,
            rows,
            count,
            i,
            rowBottom;    
            
        if (me.variableRowHeight) {
            rows = view.getNodes();
            count = store.getCount();
        
           if(rows.length > 0){
                for (i = 0; i < count; i++) {
                    rowBottom = Ext.fly(rows[i]).getOffsetsTo(view.el)[1] + rows[i].offsetHeight;


                    
                    if (rowBottom > view.el.dom.clientHeight) {
                        return;
                    }


                    if (rowBottom > 0) {
                        return i + me.tableStart;
                    }
                }
            }
        } else {
            return Math.floor(scrollTop / me.rowHeight);
        }
    },


    getLastVisibleRowIndex: function() {
        var me = this,
            store = me.store,
            view = me.view,
            clientHeight = view.el.dom.clientHeight,
            rows,
            count,
            i,
            rowTop;


        if (me.variableRowHeight) {
            rows = view.getNodes();
            count = store.getCount();
            
            if(rows.length > 0){
                for (i = count - 1; i >= 0; i--) {
                    rowTop = Ext.fly(rows[i]).getOffsetsTo(view.el)[1];


                    
                    if (rowTop < 0) {
                        return;
                    }
                    if (rowTop < clientHeight) {
                        return i + me.tableStart;
                    }
                }
            }
        } else {
            return me.getFirstVisibleRowIndex() + Math.ceil(clientHeight / me.rowHeight) + 1;
        }
    }    
});
May it help you.