FIX: Add type check for merge_channels in OPM grouping gate

Author: PragnyaKhandelwal

examples/datasets/kernel_phantom.py

@@ -106,3 +106,14 @@
 )
 mne.viz.plot_dipole_locations(dipoles=dip, mode="arrow", color=(0.2, 1.0, 0.5), fig=fig)
 mne.viz.set_3d_view(figure=fig, azimuth=30, elevation=70, distance=0.4)
+
+# %%
+# Grouped OPM topomap visualization
+# ==================================
+#
+# Since Kernel OPMs are triaxial sensors (measuring Bx, By, Bz directions),
+# we can visualize them as grouped topomaps showing radial and tangential
+# components side-by-side. This is only done when merge_channels=True
+# and there are multiple colocated channels at each location:
+
+fig = evoked.plot_topomap(times=[t_peak], merge_channels=True, sphere=sphere)

mne/viz/topomap.py

@@ -372,6 +372,10 @@ def _should_use_opm_orientation_groups(info, picks, merge_channels, ch_type):
     if not pick_chs or not all(ch["coil_type"] in _opm_coils for ch in pick_chs):
         return False
 
+    # merge_channels should be a list of overlap sets, not a boolean
+    if not isinstance(merge_channels, (list, tuple)):
+        return False
+
     return any(len(overlap_set) >= 3 for overlap_set in merge_channels)