fix: set missing import to None, ensure first frame exists before fetching

Author: mgxd

nibabel/nicom/dicomwrappers.py

@@ -467,7 +467,7 @@ def __init__(self, dcm_data):
         except TypeError:
             raise WrapperError("PerFrameFunctionalGroupsSequence is empty.")
         # DWI image where derived isotropic, ADC or trace volume was appended to the series
-        if self.frames[0].get([0x18, 0x9117]):
+        if self.frames[0] and self.frames[0].get([0x18, 0x9117], None):
             self.frames = Sequence(
                 frame for frame in self.frames if
                 frame.get([0x18, 0x9117])[0].get([0x18, 0x9075]).value

nibabel/pydicom_compat.py

@@ -21,7 +21,7 @@
 import numpy as np
 
 have_dicom = True
-pydicom = read_file = tag_for_keyword = None
+pydicom = read_file = tag_for_keyword = Sequence = None
 
 try:
     import dicom as pydicom