stitching: issue if only has one scan

Get the following issue when trigger a preview (when should raise an error instead)

ERROR:silx.gui.qt._qt:<class 'UnboundLocalError'> local variable 'scan_0' referenced before assignment   File "/scisoft/tomotools/x86_64/cuda11/test_henri_2/lib/python3.8/site-packages/tomwer/gui/stitching/stitching.py", line 845, in run
    self._output_identifier = stitcher.stitch()
  File "/scisoft/tomotools/x86_64/cuda11/test_henri_2/lib/python3.8/site-packages/nabu/stitching/z_stitching.py", line 572, in stitch
    self._check_inputs()
  File "/scisoft/tomotools/x86_64/cuda11/test_henri_2/lib/python3.8/site-packages/nabu/stitching/z_stitching.py", line 753, in _check_inputs
    if isinstance(scan_0, HDF5TomoScan) and isinstance(scan_1, HDF5TomoScan):