+ started implementing the new Draft snap

git-svn-id: https://free-cad.svn.sourceforge.net/svnroot/free-cad/trunk@5309 e8eeb9e2-ec13-0410-a4a9-efa5cf37419d

src/Mod/Draft/draftlibs/fcvec.py

@@ -174,6 +174,7 @@ def rounded(v):
 
 def getPlaneRotation(u,v,w=None):
         "returns a rotation matrix defining the (u,v,w) coordinates system"
+        if (not u) or (not v): return None
         if not w: w = u.cross(v)
         typecheck([(u,Vector), (v,Vector), (w,Vector)], "getPlaneRotation")
         m = FreeCAD.Matrix(