ImageClip's with default settings can not be concatenated

[AlexanderNickolsky]

How to reproduce:
Create two ImageClips from images. Pass them as arguments to concatenate_videoclips.
The result is an error.

The reason is somewhat complicated. The ImageClip, created with default settings, has a mask. A mask is itself a clip and has no duration attribute. These masks are concatenated using the same concatenate_videoclips functions which requires duration attribute.

Possible fix. There are many possible ways of fixing this, but the simplest is to set duration attribute of the mask clip in VideoClip.py

            elif transparent:
                self.mask = ImageClip(
                    1.0 * img[:, :, 3] / 255, ismask=True, **duration=duration** )

[ghost]

@AlexanderNickolsky .. I believe this is fixed now.. Can you reopen if, if it persists?