fix circle (#5865)

Author: massich

mne/io/edf/edf.py

@@ -281,7 +281,7 @@ def _read_segment_file(self, data, idx, fi, start, stop, cals, mult):
                     d_sidx = d_lims[ai][0]
                     d_eidx = d_lims[ai + n_read - 1][1]
                     if n_samps[ci] != buf_len:
-                        if ci in stim_channel:
+                        if stim_channel is not None and ci in stim_channel:
                             # Stim channel will be interpolated
                             old = np.linspace(0, 1, n_samps[ci] + 1, True)
                             new = np.linspace(0, 1, buf_len, False)