I have very simple XML source and OLE DB Source in Foreach Loop Container,
but seem to get error, because XML Source cannot find the XML.

"Error at Data Flow Task. The property XMLDataVariable on the XML Source was empty"

Variables:
FileExtention = *.xml
FilePath =
FolderPath = C:\Customer\XML

XML Source:
Data Access mode = XML file from variable
Variable name = User::filePath

Foreach Loop Container:
Enumerator = Foreach File Enumerator
Expression-Directory = @User:FolderPath
Expression-FileSpec = @User::FileExtension
Enumerator Folder = C:\Customer\XML
Files = *.xml
Variable mapping-User::FilePath = 0

Have you filled the variable (filePath) with a default value? Otherwise the XML Source cannot validate. Alternative is look for a property "DelayValidation". In most cases this is a property of the connection manager, but you're not using one for the xml

I added default filepath+filename to variable FilePath. It works now without error, but in reality I don't know what is exact name of file.

How to make filepath variable for XML Source working without defining default filepath?

Try setting the DelayValidation property on the Data Flow Task that contains the xml source.