Grease Pencil: Fix python errors opening N panel -> GP with empty VSE

Solves errors, but not sure interface is indeed what users will expect.

Will ask GP team to check on this.

release/scripts/startup/bl_ui/properties_grease_pencil_common.py

@@ -61,6 +61,9 @@ def gpencil_stroke_placement_settings(context, layout):
 
 def gpencil_active_brush_settings_simple(context, layout):
     brush = context.active_gpencil_brush
+    if brush is None:
+        layout.label("No Active Brush")
+        return
 
     col = layout.column()
     col.label("Active Brush:      ")