wxGUI/lmgr: fix launch GCP Manager if not map display is activated

[tmszi]

Describe the bug
When no map display is activated GCP Manager doesn't launch.

To Reproduce
Steps to reproduce the behavior:

1. Launch wxGUI
2. Close all activated map display
3. From the main wxGUI window toolbar launch GCP Manager
4. See error

Traceback (most recent call last):
  File "/usr/lib64/grass83/gui/wxpython/lmgr/frame.py", line
737, in OnGCPManager

GCPWizard(self, self._giface)
  File "/usr/lib64/grass83/gui/wxpython/gcp/manager.py",
line 164, in __init__

self.mappage = DispMapPage(self.wizard, self)
  File "/usr/lib64/grass83/gui/wxpython/gcp/manager.py",
line 833, in __init__

extraItems=self.GetSelectTargetRasterExtraItems(),
  File "/usr/lib64/grass83/gui/wxpython/gcp/manager.py",
line 1037, in GetSelectTargetRasterExtraItems

return {self.web_servc_lyrs_root_node_name:
self.GetWebServiceLayers().keys()}
  File "/usr/lib64/grass83/gui/wxpython/gcp/manager.py",
line 1028, in GetWebServiceLayers

for layer in self.parent._giface.GetLayerList():
  File "/usr/lib64/grass83/gui/wxpython/lmgr/giface.py",
line 64, in __iter__

item = self._tree.GetFirstChild(self._tree.root)[0]
AttributeError
:
'NoneType' object has no attribute 'GetFirstChild'

Expected behavior
When no map display is activated GCP Manager should be launch without error.

System description (please complete the following information):

• Operating System: all
• GRASS GIS version: all

[petrasovaa]

Thanks!