Bug 202254 Exception is thrown when category values are null and type of X axis is set to DateTime
chart/org.eclipse.birt.chart.engine/src/org/eclipse/birt/chart/util/CDateTime.java
cPrev = (Calendar) dsi.last( );
			dsi.reset( );
			while ( dsi.hasNext( ) )
			{
				cCurr = (Calendar) dsi.next( );
				if ( cCurr != null
						&& cPrev != null
						&& cCurr.get( iaUnitTypes[k] ) != cPrev.get( iaUnitTypes[k] ) )
				{
					return iaUnitTypes[k]; 
					
				}
				if ( cCurr != null )
				{
					cPrev = cCurr;
				}
			}
		}
		
		return Calendar.DATE;
	}
	/**
	 * Walks through all values in a datetime array and computes the least
	 * significant unit for which a difference was noted.
	 * 
	 * @param cdta
	 *            A datetime array for which the least significant unit
	 *            difference is to be computed
	 * 
	 * @return The least significant unit for which a difference in datetime