To replicate the error:
- Make two objects, select them, pick one option from the Bool Tool Panel > Brush Boolean (for instance Difference)
- With the two objects still selected go to Properties > Canvas > Apply All
- Select the brush object (it's drawn as a bounding box in the 3D view) go to Properties > Brush > Remove Brush
scripts\addons\object_boolean_tools.py", line 263, in Remove for mod in Canvas.modifiers: AttributeError: 'NoneType' object has no attribute 'modifiers'
Seems that since the brushes from the Canvas being applied there's nothing to cycle through the loop of the removal function. The remove the brush object, it needs to be deleted manually.