More friendly exception when datasource empty

[andreykhr]

Summary of the issue

Environment

The in-use version:
Operating system:  OS X 10.14.2
IDE: Rider 2018.1

Example code/Steps to reproduce:

Based on example https://github.com/VahidN/PdfReport.Core/blob/master/src/PdfRpt.Core.FunctionalTests/IListPdfReport.cs

If comment datasource line №79 is fails on null reference exception.

Suppose it must throw some another exceptions. More understandable, something like 'datasource' not configured properly or datasource is empty.

            .MainTableDataSource(dataSource =>
            {
                var listOfRows = new List<User>
                {
                    new User {Id = 0, LastName = "Test Degree Sign: 120°", Name = "Celsius", Balance = 0}
                };

                for (var i = 1; i <= 200; i++)
                {
                    listOfRows.Add(new User { Id = i, LastName = "LastName " + i, Name = "Name " + i, Balance = i + 1000 });
                }
               // dataSource.StronglyTypedList(listOfRows);
            }) 

another one

//            .MainTableDataSource(dataSource =>
//            {
//                var listOfRows = new List<User>
//                {
//                    new User {Id = 0, LastName = "Test Degree Sign: 120°", Name = "Celsius", Balance = 0}
//                };

//                for (var i = 1; i <= 200; i++)
//                {
//                    listOfRows.Add(new User { Id = i, LastName = "LastName " + i, Name = "Name " + i, Balance = i + 1000 });
//                }
//                dataSource.StronglyTypedList(listOfRows);
//            }) 

Output:

Exception message: 
System.NullReferenceException : Object reference not set to an instance of an object.
Full Stack trace:
System.NullReferenceException : Object reference not set to an instance of an object.
   at PdfRpt.Core.PdfTable.RenderMainTable.setLocalCache()
   at PdfRpt.Core.PdfTable.RenderMainTable.initData()
   at PdfRpt.Core.PdfTable.RenderMainTable.AddToDocument()
   at PdfRpt.PdfReportDocument.createPdf()
   at PdfRpt.PdfReportDocument.GeneratePdf()
   at PdfRpt.FluentInterface.PdfReport.Generate(Action`1 pdfRptFileBuilder)
   at Tests.Tests.CreateIListPdfReport() in /Users/andreykhr/src/KhramtSoft/PdfFixGen/IssueTest/TestProject1/UnitTest1.cs:line 25
   at Tests.Tests.Test1() in /Users/andreykhr/src/KhramtSoft/PdfFixGen/IssueTest/TestProject1/UnitTest1.cs:line 19

[VahidN]

fixed via #84b1821

[lock]

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