micro2600

``` Caused by: java.lang.NullPointerException: Cannot invoke "java.awt.Color.getRGB()" because "color" is null at org.ttzero.excel.entity.style.ColorIndex.indexOf(ColorIndex.java:54) at org.ttzero.excel.entity.style.Border.writeProperties(Border.java:561) at org.ttzero.excel.entity.style.Border.toDom(Border.java:478) at org.ttzero.excel.entity.style.Styles.writeTo(Styles.java:568) at org.ttzero.excel.entity.e7.XMLWorkbookWriter.writeGlobalAttribute(XMLWorkbookWriter.java:181) at org.ttzero.excel.entity.e7.XMLWorkbookWriter.createTemp(XMLWorkbookWriter.java:416) at org.ttzero.excel.entity.e7.XMLWorkbookWriter.writeTo(XMLWorkbookWriter.java:124) at org.ttzero.excel.entity.Workbook.writeTo(Workbook.java:712) ```