have you created your media types on in your upgraded site yet?

http://world.episerver.com/Documentation/Items/Upgrading/EPiServer-CMS/75/Migrating-VPP-based-files-to-the-new-media-system/

I just created an image type and generic media type

Sorry I forgot to migrate the old site first to 7.5 via Deployment Center. Now I encounter a different error message:

Could not load file or assembly 'EPiServer.Cms.Shell.UI, Version=2.1.82.0, Culture=neutral, PublicKeyToken=8fe83dea738b45b7' or one of its dependencies. The system cannot find the file specified.



   at System.Reflection.RuntimeAssembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks)
   at System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef, Evidence assemblySecurity, RuntimeAssembly reqAssembly, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks)
   at System.Reflection.RuntimeAssembly.InternalLoad(String assemblyString, Evidence assemblySecurity, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean forIntrospection)
   at System.Reflection.RuntimeAssembly.InternalLoad(String assemblyString, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection)
   at System.Reflection.Assembly.Load(String assemblyString)
   at System.UnitySerializationHolder.GetRealObject(StreamingContext context)

   at VppMigrationTool.Util.SiteProxy.Initialize(String destinationPath, String virtualDirectory)
   at VppMigrationTool.Converter.get_RuntimeProxyWrapper() in d:\Builds\132\EPiServerCMS Modules\VppMigrationTool\Sources\C\TFS\VppMigrationTool\Converter.cs:line 52
   at VppMigrationTool.Converter.FillSiteData() in d:\Builds\132\EPiServerCMS Modules\VppMigrationTool\Sources\C\TFS\VppMigrationTool\Converter.cs:line 101



--- Inner exception ---



The system cannot find the file specified. (Exception from HRESULT: 0x80070002)



   at System.Reflection.RuntimeAssembly.nLoadFile(String path, Evidence evidence)
   at System.Reflection.Assembly.LoadFile(String path)
   at VppMigrationTool.Converter.CurrentDomain_AssemblyResolve(Object sender, ResolveEventArgs args) in d:\Builds\132\EPiServerCMS Modules\VppMigrationTool\Sources\C\TFS\VppMigrationTool\Converter.cs:line 40
   at System.AppDomain.OnAssemblyResolveEvent(RuntimeAssembly assembly, String assemblyFullName)

Make sure you have the latest version of the EPiServer UI from the add-on store. 
Optionally, install a fresh alloy site, and copy the files from appdata\modules and appdata\modulesrepository, along with wwwroot\modulesbin

Hi,

I am also facing the same above issue , while migrating images and files from Episerve 6R2 to Episerver 7.0.586.1

Object reference not set to an instance of an object.at VppMigrationTool.Util.SiteProxy.InitalizeEPiServer(String webConfigPath) in e:\Sotwares\vppmigration-source-code\VPPMigration\VppMigrationTool\Util\SiteProxy.cs:line 45

Kindly suggest