fix of the possible null reference error in the metadata creator

src/assembly/Comprehensive/MsImaging/Metadata.vb

@@ -45,13 +45,18 @@ Namespace MsImaging
 
         <MethodImpl(MethodImplOptions.AggressiveInlining)>
         Public Function GetMetadata() As Dictionary(Of String, String)
-            Return New Dictionary(Of String, String) From {
+            Dim datalist As New Dictionary(Of String, String) From {
                 {"width", scan_x},
                 {"height", scan_y},
-                {"resolution", resolution},
-                {"mzmin", mass_range.Min},
-                {"mzmax", mass_range.Max}
+                {"resolution", resolution}
             }
+
+            If Not mass_range Is Nothing Then
+                datalist!mzmin = mass_range.Min
+                datalist!mzmax = mass_range.Max
+            End If
+
+            Return datalist
         End Function
 
     End Class

src/assembly/Comprehensive/MsImaging/MsImagingRaw.vb

@@ -59,6 +59,7 @@ Imports BioNovoGene.Analytical.MassSpectrometry.Assembly.mzData.mzWebCache
 Imports BioNovoGene.Analytical.MassSpectrometry.Math.Spectra
 Imports Microsoft.VisualBasic.CommandLine.InteropService.Pipeline
 Imports Microsoft.VisualBasic.ComponentModel.Ranges.Model
+Imports Microsoft.VisualBasic.DataStorage.netCDF.Components
 Imports Microsoft.VisualBasic.Imaging.Math2D
 Imports Microsoft.VisualBasic.Language
 Imports Microsoft.VisualBasic.Linq
@@ -131,6 +132,9 @@ Namespace MsImaging
             Dim pixels As New List(Of ScanMS1)
             Dim cutoff As New RelativeIntensityCutoff(intocutoff)
             Dim metadata As New Metadata
+            Dim mzmin As New List(Of Double)
+            Dim mzmax As New List(Of Double)
+            Dim mzvals As Double()
 
             If progress Is Nothing Then
                 progress = Sub(msg)
@@ -141,12 +145,19 @@ Namespace MsImaging
             ' each row is a small sample in current sample batch
             For Each row As mzPack In src
                 pixels += row.MeasureRow(yscale, correction, cutoff, sumNorm, labelPrefix, progress)
+                mzvals = row.MS.Select(Function(a) a.mz).IteratesALL.ToArray
+
+                If mzvals.Length > 0 Then
+                    mzmin.Add(mzvals.Min)
+                    mzmax.Add(mzvals.Max)
+                End If
             Next
 
             Dim polygon As New Polygon2D(pixels.Select(Function(scan) scan.GetMSIPixel))
 
             metadata.scan_x = polygon.xpoints.Max
             metadata.scan_y = polygon.ypoints.Max
+            metadata.mass_range = New DoubleRange(mzmin.Min, mzmax.Max)
 
             Return New mzPack With {
                 .MS = pixels.ToArray,