Fixes a NullPointerException in Chart.java when enable/disable scroll.

While working in my project with this (awesome) library I encountered a NullPointerException in the enableScroll-method in the Chart.java. 

It occurred in the following situation: I had a BarChart displayed in the header of a ListView, then I updated the ListView adapter at run-time in response to an OnChartValueSelectedListener of the bar chart. Thereafter the error occurs, i.e. the ViewParent-object was null.

Adding this try-catch structure fixed the problems for me and I didn't see any strange behavior. I also added this try-catch structure to the disableScroll method, because the two methods are very identical.

Do you think this try-catch structure is sufficient ? Or are there extra changes necessary because the enableScroll method 'fails'.

You can also check this behaviour in my project : https://github.com/MPieter/Notification-Analyser . Comment these changes and then see what happens.