that's helpful as it isolates that the problem exists in LoadFile - looking at the code I see this as
https://raw.githubusercontent.com/dnnsoftware/Dnn.Platform/master/DNN%20Platform/DotNetNuke.Web/UI/WebControls/DnnFilePickerUploader.cs
Looking at the code the LoadFiles method looks the likely culprit where a null reference exception could occur i.e. the function expects a select folder, but if no folder is selected (e.g. in the list of folders failed or was empty). However the only way I can see this happening is if if there was a problem with folder permissions.
Can you check this by running the following query (either in sql server management studio or host->sql). Note: the first value (0) is the portalid, so make sure that matches your portal id, the third value (1) is the userid so make sure you're using the userid of your host/admin user.
GetFoldersByPermissions 0, 'READ,ADD', 1 ,-1, ''
This should return a set of results similar to this -but if your query is not returning results that would isolate where the problem is
FolderID PortalID FolderPath StorageLocation IsProtected IsCached LastUpdated CreatedByUserID CreatedOnDate LastModifiedByUserID LastModifiedOnDate UniqueId VersionGuid FolderMappingID ParentID IsVersioned WorkflowID MappedPath
46 0 0 1 0 NULL -1 2014-03-21 21:19:19.113 -1 2014-03-21 21:19:19.627 C0F8145F-6050-4D88-A500-DBD682D3A166 EC743024-B822-45E0-9084-922FD143821D 8 NULL 0 NULL
47 0 Cache/ 0 0 0 NULL -1 2014-03-21 21:19:19.180 -1 2014-03-21 21:19:19.213 F9FD2F5A-565D-49CC-85BA-11C45ABB3B12 A4252E38-6F85-410A-AF27-C855909EFFEF 8 46 0 NULL Cache/
48 0 Cache/Pages/ 0 0 0 NULL -1 2014-03-21 21:19:19.213 -1 2014-03-21 21:19:19.230 B4D16284-B066-4C45-B85C-73DED3208D28 A2DC5FB5-14AC-4CE5-9594-854047AA85F4 8 47 0 NULL Cache/Pages/
49 0 Images/ 0 0 0 NULL -1 2014-03-21 21:19:19.237 -1 2014-03-21 21:19:19.563 4B3374F7-C288-477D-A863-98978B8DA428 39F0EA04-91F8-446D-A8CE-CE15DFADDF44 8 46 0 NULL Images/
50 0 Images/DNN/ 0 0 0 NULL -1 2014-03-21 21:19:19.560 -1 2014-03-21 21:19:19.590 087F4DA9-1184-4034-B16C-89D8D8B3E199 9CB5E129-7733-4F79-BD60-050BAEDA85EA 8 49 0 NULL Images/DNN/
51 0 Images/DNN/Logos/ 0 0 0 NULL -1 2014-03-21 21:19:19.587 -1 2014-03-21 21:19:19.593 AB26E01B-4498-4F49-A3F5-4319000E3C33 321D58CB-672A-4A50-8FFF-5A367DC4CE01 8 50 0 NULL Images/DNN/Logos/
52 0 Templates/ 0 0 0 NULL -1 2014-03-21 21:19:19.600 -1 2014-03-21 21:19:19.607 90C188AE-D64A-45A2-8826-9414D682CAF7 428ACDC2-C899-4E17-A949-0E1871A629A1 8 46 0 NULL Templates/
53 0 Users/ 0 1 0 NULL -1 2014-03-21 21:19:19.623 -1 2014-03-21 21:19:19.630 EABA4271-5A00-4AFB-8DCC-395798C54E26 2C2B7888-1AB4-4942-9487-7D653AB1A510 8 46 0 NULL Users/