Sitecore: Value cannot be null. Parameter name: uri - sitecore

Question

Exception: System.ArgumentNullException
Message: Value cannot be null.
Parameter name: uri
Source: Sitecore.Kernel
I was editing a placeholder item in a local
Sitecore 8.1 (rev. 151207) instance when I got this error. Now I can't edit, delete, or reset it (or its parent folders). I also can't read the item in the content editor as the error pops up when I click on the parent folder. I've tried deleting/editing it and it's parent folder from the content editor, through code, and through PowerShell commands but the same error always shows.
I've also tried replacing my configs and bin folders with my backup copies, clearing cache, restarting IIS and the app pool, but nothing has helped.
I'm completely stuck. Does anyone know how to fix this?
Edit:
The full error in the logs is:
Exception: System.ArgumentNullException
Message: Value cannot be null.
Parameter name: uri
Source: Sitecore.Kernel
at Sitecore.Diagnostics.Assert.ArgumentNotNull(Object argument, String argumentName)
at Sitecore.Data.ItemUri..ctor(ItemUri uri)
at Sitecore.Data.Items.Item.GetItemUriFromSourceItem()
at Sitecore.Data.Items.Item.get_SourceUri()
at Sitecore.Data.Items.Item.get_Source()
at Sitecore.Data.Fields.Field.GetSourceItem()
at Sitecore.Data.Fields.Field.GetInheritedValue(Boolean allowStandardValue)
at Sitecore.Pipelines.GetFieldValue.GetInheritedValue.Process(GetFieldValueArgs args)
at (Object , Object[] )
at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
at Sitecore.Data.Fields.Field.GetValue(Boolean allowStandardValue, Boolean allowDefaultValue, Boolean allowFallbackValue, Boolean allowInheritValue, Boolean allowInnerValue)
at Sitecore.Pipelines.ItemProvider.GetItem.GetLanguageFallbackItem.IsItemFallbackEnabled(Item item)
at Sitecore.Pipelines.ItemProvider.GetItem.GetLanguageFallbackItem.Process(GetItemArgs args)
at (Object , Object[] )
at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
at Sitecore.Data.Managers.PipelineBasedItemProvider.ExecuteAndReturnResult[TArgs,TResult](String pipelineName, String pipelineDomain, Func`1 pipelineArgsCreator, Func`1 fallbackResult)
at Sitecore.Data.Managers.ItemManager.GetItem(ID itemId, Language language, Version version, Database database, SecurityCheck securityCheck)
at Sitecore.Nexus.Data.DataCommands.GetChildrenCommand.Execute(Item )
at Sitecore.Data.Engines.EngineCommand`2.Execute()
at Sitecore.Data.Managers.ItemProvider.GetChildren(Item item, ChildListOptions options)
at Sitecore.Data.Managers.ItemProvider.GetChildren(Item item, SecurityCheck securityCheck, ChildListOptions options)
at Sitecore.Data.Managers.PipelineBasedItemProvider.ExecuteAndReturnResult[TArgs,TResult](String pipelineName, String pipelineDomain, Func`1 pipelineArgsCreator, Func`1 fallbackResult)
at Sitecore.Data.Managers.PipelineBasedItemProvider.GetChildren(Item item, SecurityCheck securityCheck)
at Sitecore.Collections.ChildList.Populate(ChildListOptions options)
at Sitecore.Shell.Framework.Pipelines.DeleteItems.GetItemClones(Item item, Boolean processChildren)
at Sitecore.Shell.Framework.Pipelines.DeleteItems.GetItemClones(Item item, Boolean processChildren)
at Sitecore.Shell.Framework.Pipelines.DeleteItems.GetItemClones(List`1 items)
at Sitecore.Shell.Framework.Pipelines.DeleteItems.GetCheckCloneLinksMessage(List`1 items)
at Sitecore.Shell.Framework.Pipelines.DeleteItems.CheckCloneLinks(ClientPipelineArgs args)

Answer 1

The exception you provided occurs because you have nonempty invalid value in the __Source Item - {19B597D3-2EDD-4AE2-AEFE-4A94C7F10E31} field in your item. __Source Item is responsible for storing the original item URI for the clone items.
I don't know how you achieved that because there is a processor ValidateCloneSourceModifications in the saveItem pipeline that prevents of saving items with invalid __Source Item field.
I was able to reproduce your exception by changing the field value in the DB.
How to fix:
Find you broken field in the master DB -> SharedFields table. The FieldId is '19B597D3-2EDD-4AE2-AEFE-4A94C7F10E31'.
Fix it. It should be something like sitecore://master/{AE76A034-9491-4B83-99F5-39F227D6FB59}, where the ID is the original item id. You could also just set '' as a value, but it will remove the binding to the original item.
Clear cache.
It must help.

Answer 2

You can try the DB Browser /sitecore/admin/dbbrowser.aspx with this tool you can also delete items.
If this DB Browser admin tools fail, then I'm curious what's in the master database, check directly in sql what make this item special is than an option.