SAR calculation gives error due to missing RF signal

[h3lg3]

Describe the bug
Using the example sequence write_epi.py for the SAR calculation with the pypulseq.SAR.SAR_calc.cal_SAR(seq) function results in an error.

"SAR_calc.py", line 107, in _SAR_from_seq
signal = rf.signal
AttributeError: 'NoneType' object has no attribute 'signal'

To Reproduce

from write_epi import main as write_epi
import pypulseq as pp 

write_epi(plot = False, write_seq = True, seq_filename = 'epi_pypulseq.seq')

pp.SAR.SAR_calc.calc_SAR('epi_pypulseq.seq')

I would propose to check in addition to line 105 of SAR_calc.py if the signal of the rf event is not none.
Instead of

if hasattr(block, "rf"):  # has rf
    rf = block.rf
    signal = rf.signal

use

if hasattr(block, "rf"):  # has rf 
    rf = block.rf
    if rf is not None:  # rf signal is not None
        signal = rf.signal

However, I have the feeling that the rf block is always present in the current version of pypulseq, in that case the first if statement is obsolete, if that is not the case please correct me.

Kind regards
Helge