fix fork stripping + fork None values

src/kyupy/sim.py

@@ -229,6 +229,7 @@ class SimOps:
                     prev_line = prev_line.driver.ins[0]
                 stem_idx = prev_line.index
                 for ol in f.outs:
+                    if ol is not None:
                         stems[ol] = stem_idx
 
         # calculate level (distance from PI/PPI) and reference count for each line