Wrap the data in a method that handles the formatting.
For example, change:
dreader("AZmin").ToString()
To:
Function FormatData(value As Double?) As String
If value Is Nothing Then
Return ""
End If
Return value.ToString()
End Function
Where
value_type
is the filed value type, eg:
double
,
float
,
decimal
, etc...
Then to use:
dgvProduktion.Rows.Add(..., FormatData(dreader("AZmin")) ,...)