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.

Written by the indexing model from the issue text.