Fixed IndexOutOfBoundsException in MaterialCollapsible

[dieterblancke]

This should fix an IndexOutOfBoundsException that occurs when executing setActive with an index equal to the amount of widgets.

This error started occuring since this commit afaik: 60d3d21

[dieterblancke]

@kevzlou7979 if you are able to, please clarify as to what the intention is here.

From the issue #998 it seems like it's now intended to have a 0-based index, yet the log message is still there and there is a check for > 0.

Yet by allowing index <= getWidgetCount(), it can also have an IndexOutOfBoundsException when the index == count (the issue we're running into).

It's fixable by either changing the index <= getWidgetCount() to index < getWidgetCount() and changing the index > 0 to index >= 0 or by reverting to a 1-based index.

Feel free to clarify, and I will update the pull request accordingly 😊