If you are upgrading from version 5.3.1 or older to version 5.4.0 please note the following changes:
All BluetoothHelper class events now have BluetoothHelper as their 1st parameter.
this is for better support of connection to multiple instance,
for example, previously you would do:
bluetoothHelper.OnConnected += () 
{
    //do something...
}
now you do :
bluetoothHelper.OnConnected += (helper) 
{
    //do something...
    //inside this function, you now have a reference to the caller of that function!
}
the above is applicable for all events.
another exampler:
bluetoothHelper.OnCharacteristicChanged += OnCharacteristicChanged;
void OnCharacteristicChanged (BluetoothHelper helper, byte[] value, BluetoothHelperCharacteristic characteristic)
{
    //do somthing
}
previously it was : 
void OnCharacteristicChanged (byte[] value, BluetoothHelperCharacteristic characteristic)
{
    //do somthing
}