Fix T59396 Grease pencil brush crash

The problem was the brush was not initializated because some code was missing (marked as ToDo)

release/scripts/startup/bl_ui/space_view3d_toolbar.py

@@ -1485,12 +1485,13 @@ class VIEW3D_PT_tools_grease_pencil_brush(View3DPanel, Panel):
 
         col = row.column()
         brush = context.active_gpencil_brush
-        gp_settings = brush.gpencil_settings
 
         sub = col.column(align=True)
         sub.operator("gpencil.brush_presets_create", icon='HELP', text="")
 
         if brush is not None:
+            gp_settings = brush.gpencil_settings
+
             # XXX: Items in "sub" currently show up beside the brush selector in a separate column
             if brush.gpencil_tool == 'ERASE':
                 sub.prop(gp_settings, "use_default_eraser", text="")

source/blender/editors/sculpt_paint/paint_ops.c

@@ -124,7 +124,8 @@ static int brush_add_gpencil_exec(bContext *C, wmOperator *UNUSED(op))
 
 	BKE_paint_brush_set(paint, br);
 
-	/* TODO init grease pencil specific data */
+	/* init grease pencil specific data */
+	BKE_brush_init_gpencil_settings(br);
 
 	return OPERATOR_FINISHED;
 }