Pymca main window: avoid pop-up on broken link

[woutdenolf]

import h5py

with h5py.File("test.h5", track_order=True, mode="w") as fh:
    i = 1
    entry = fh.create_group(f"1.{i}")
    entry["instrument/detector/data"] = [1, 2, 3]
    entry["measurement/data"] = h5py.SoftLink(f"/1.{i}/instrument/detector/data")

    i = 2
    entry = fh.create_group(f"1.{i}")
    entry["measurement/data"] = h5py.SoftLink(f"/1.{i}/instrument/detector/data")

A dialog pops up with

'NoneType' object has no attribute '_posixPath'   File "/home/denolf/dev/pymca/src/PyMca5/PyMcaGui/io/hdf5/HDF5Widget.py", line 562, in rowCount
    return len(self.getProxyFromIndex(index))
  File "/home/denolf/dev/pymca/src/PyMca5/PyMcaGui/io/hdf5/HDF5Widget.py", line 354, in __len__
    return len(self.children)
  File "/home/denolf/dev/pymca/src/PyMca5/PyMcaGui/io/hdf5/HDF5Widget.py", line 207, in children
    finalList[i][1]._posixPath = posixpath.join(self.name,

[vasole]

As far as a broken link icon is shown somewhere...