Error in plot3d.py example

Hi,
I recently installed arrayfire python bindings. It passes all the tests and it launches few of the examples. However I got the following error when I tried to launch `plot3d.py` in the `examples/graphics` folder. Any idea ?

```python
ArrayFire v3.6.2 (CUDA, 64-bit Linux, build dc38ef1)
Platform: CUDA Toolkit 10, Driver: 418.39
[0] Quadro M2000M, 4044 MB, CUDA Compute 5.0
Traceback (most recent call last):
File "examples/graphics/plot3.py", line 32, in
win.plot3(Pts)
File "/home/user1/miniconda3/envs/ptmviewer/lib/python3.7/site-packages/arrayfire-3.6.20181017-py3.7.egg/arrayfire/graphics.py", line 296, in plot3
assert(line.numdims() == 3)
AttributeError: 'NoneType' object has no attribute 'numdims'
```

Reproduce the failure by running examples/graphics/plot3.py, then inspect arrayfire/graphics.py around line 296 where plot3 calls numdims() on line. Confirm the example completes without the reported NoneType AttributeError and that the 3D plot is launched successfully.

索引モデルが issue の本文から書いたものです。