pretty_print TypeError when using ek+raw mode

[miaotony]

Describe the bug
config: use_ek=True, include_raw=True
Pyshark raises TypeError: 'NoneType' object is not iterable when calling packet.pretty_print() or packet.__str__() function.

To Reproduce

import pyshark

def tshark_callback(packet):
    packet.pretty_print()
    # print(packet.__str__())

cap = pyshark.LiveCapture(use_ek=True, include_raw=True, interface='wlan')
cap.set_debug()
cap.apply_on_packets(tshark_callback)

Run the code and surf on the Internet for some time, the program exit with the following error.

...
Layer TLS:
Layer FRAME_RAW:
Layer ETH_RAW:
Layer IP_RAW:
Layer TCP_RAW:
Layer TCP_TCP_SEGMENTS_RAW:
Layer TCP_TCP_SEGMENTS:
2022-08-20 03:28:44,374 - LiveCapture - DEBUG - Cleanup Subprocess (pid 18028)
2022-08-20 03:28:44,395 - LiveCapture - DEBUG - Cleanup Subprocess (pid 26176)
Traceback (most recent call last):
  File "test.py", line 9, in <module>
    cap.apply_on_packets(tshark_callback)
  File "D:\Programs\Anaconda\lib\site-packages\pyshark\capture\capture.py", line 256, in apply_on_packets
    return self.eventloop.run_until_complete(coro)
  File "D:\Programs\Anaconda\lib\asyncio\base_events.py", line 584, in run_until_complete
    return future.result()
  File "D:\Programs\Anaconda\lib\site-packages\pyshark\capture\capture.py", line 267, in packets_from_tshark
    await self._go_through_packets_from_fd(tshark_process.stdout, packet_callback, packet_count=packet_count)
  File "D:\Programs\Anaconda\lib\site-packages\pyshark\capture\capture.py", line 294, in _go_through_packets_from_fd
    packet_callback(packet)
  File "test.py", line 4, in tshark_callback
    packet.pretty_print()
  File "D:\Programs\Anaconda\lib\site-packages\pyshark\packet\packet.py", line 114, in pretty_print
    layer.pretty_print()
  File "D:\Programs\Anaconda\lib\site-packages\pyshark\packet\layers\base.py", line 59, in pretty_print
    self._pretty_print_layer_fields(writer)
  File "D:\Programs\Anaconda\lib\site-packages\pyshark\packet\layers\ek_layer.py", line 110, in _pretty_print_layer_fields
    for field_name in self.field_names:
  File "D:\Programs\Anaconda\lib\site-packages\pyshark\packet\layers\ek_layer.py", line 65, in field_names
    return list({field_name.split("_", 1)[0] for field_name in self.all_field_names})
  File "D:\Programs\Anaconda\lib\site-packages\pyshark\packet\layers\ek_layer.py", line 71, in all_field_names
    for field_name in self._fields_dict:
TypeError: 'NoneType' object is not iterable

Versions (please complete the following information):

• OS: Windows 10
• pyshark version: v0.5.3
• tshark version: TShark (Wireshark) 3.6.6 (v3.6.6-0-g7d96674e2a30)

[miaotony]

The bug also occurs on Linux (Ubuntu 20.04) with pyshark v0.5.3 and TShark (Wireshark) 3.2.3 (Git v3.2.3 packaged as 3.2.3-1).

Layer FRAME_RAW:
Layer ETH_RAW:
Layer IP_RAW:
Layer TCP_RAW:
Layer TCP_TCP_SEGMENTS_RAW:
Layer TCP_TCP_SEGMENTS:
2022-08-19 19:20:43,419 - LiveCapture - DEBUG - Cleanup Subprocess (pid 11566)
2022-08-19 19:20:43,424 - LiveCapture - DEBUG - Cleanup Subprocess (pid 11538)
Traceback (most recent call last):
  File "/workspaces/test.py", line 9, in <module>
    cap.apply_on_packets(tshark_callback)  
  File "/opt/python/3.10.4/lib/python3.10/site-packages/pyshark/capture/capture.py", line 256, in apply_on_packets
    return self.eventloop.run_until_complete(coro)
  File "/opt/python/3.10.4/lib/python3.10/asyncio/base_events.py", line 646, in run_until_complete
    return future.result()
  File "/opt/python/3.10.4/lib/python3.10/site-packages/pyshark/capture/capture.py", line 267, in packets_from_tshark
    await self._go_through_packets_from_fd(tshark_process.stdout, packet_callback, packet_count=packet_count)
  File "/opt/python/3.10.4/lib/python3.10/site-packages/pyshark/capture/capture.py", line 294, in _go_through_packets_from_fd
    packet_callback(packet)
  File "/workspaces/test.py", line 4, in tshark_callback
    packet.pretty_print()
  File "/opt/python/3.10.4/lib/python3.10/site-packages/pyshark/packet/packet.py", line 114, in pretty_print
    layer.pretty_print()
  File "/opt/python/3.10.4/lib/python3.10/site-packages/pyshark/packet/layers/base.py", line 59, in pretty_print
    self._pretty_print_layer_fields(writer)
  File "/opt/python/3.10.4/lib/python3.10/site-packages/pyshark/packet/layers/ek_layer.py", line 110, in _pretty_print_layer_fields
    for field_name in self.field_names:
  File "/opt/python/3.10.4/lib/python3.10/site-packages/pyshark/packet/layers/ek_layer.py", line 65, in field_names
    return list({field_name.split("_", 1)[0] for field_name in self.all_field_names})
  File "/opt/python/3.10.4/lib/python3.10/site-packages/pyshark/packet/layers/ek_layer.py", line 71, in all_field_names
    for field_name in self._fields_dict:
TypeError: 'NoneType' object is not iterable