Add Show-MappingTab function to manage tab selection and improve error handling

Start-SPMigrationGUI.ps1

@@ -1047,6 +1047,29 @@ function Set-MigrationUiBusy {
     }
 }
 
+function Show-MappingTab {
+    if ($null -eq $script:MainTabControl -or $null -eq $script:tabMapping) {
+        return
+    }
+
+    if ($script:MainTabControl -isnot [System.Windows.Forms.TabControl] -or $script:tabMapping -isnot [System.Windows.Forms.TabPage]) {
+        return
+    }
+
+    if ($null -ne $script:MainForm -and $script:MainForm.IsDisposed) {
+        return
+    }
+
+    try {
+        if ($script:MainTabControl.TabPages.Contains($script:tabMapping)) {
+            $script:MainTabControl.SelectTab($script:tabMapping)
+        }
+    }
+    catch {
+        Write-UILog -Message ("Konnte Mapping-Tab nicht aktivieren: {0}" -f $_.Exception.Message) -Level "WARN"
+    }
+}
+
 function Reset-MigrationStreamPositions {
     $script:MigrationStreamPositions = @{
         Output = [long]0
@@ -2400,7 +2423,7 @@ function Invoke-ExportFromGui {
                 Load-MappingTableFromPath -Path $mappingPath
                 Apply-ColumnDefaultsIfAvailable
                 Save-MappingTableToPath -Path $mappingPath
-                $script:MainTabControl.SelectedTab = $script:tabMapping
+                Show-MappingTab
             }
         }.GetNewClosure()
 
@@ -2798,7 +2821,7 @@ $btnLoadMapping.Add_Click({
 
         if ($dialog.ShowDialog() -eq [System.Windows.Forms.DialogResult]::OK) {
             Load-MappingTableFromPath -Path $dialog.GetPath()
-            $script:MainTabControl.SelectedTab = $script:tabMapping
+            Show-MappingTab
         }
     }
     catch {
@@ -2923,3 +2946,4 @@ if ($NoGui.IsPresent) {
 
 $formBuilder.Show()
 
+