Latest version throwing an exception in ExcelWorkSheet.cs method SavePivotTables when using a template with pivot tables.
problem line
```
var ws = Workbook.Worksheets[pt.CacheDefinition.SourceRange.WorkSheet];
```
SourceRange is null in the Sheet I am saving.
Comments: I have the same issue with EPPLUS 4.0.4 when i try to save the file. I created it with a template that contains a pivot table which source is a dynamic named range. If I use the DLL 3.1.3, I can save the file with no problem.
problem line
```
var ws = Workbook.Worksheets[pt.CacheDefinition.SourceRange.WorkSheet];
```
SourceRange is null in the Sheet I am saving.
Comments: I have the same issue with EPPLUS 4.0.4 when i try to save the file. I created it with a template that contains a pivot table which source is a dynamic named range. If I use the DLL 3.1.3, I can save the file with no problem.