macOS Xcode Errors

Every time I try to build (any) project, I get the error message:

Uncaught Exception:
*** -[NSCFDictionary setObject:forKey:]: attempt to insert nil key

Stack Backtrace:
The stack backtrace has been logged to the console.

with options to Continue (does nothing) or Quit

I have tried trashing preferences (no difference), and am running 2.4.1

Any ideas?

Current projects are Java and were building fine before.

As far as I can see this is all languages. Creating a brand new c++ tool with the default code "Hello World!" code causes the same error. The same is true for a java tool, Cocoa project - I could try more but I am getting the picture.

A reinstall is the next step, I suppose ...

... which hasn't fixed it.

Uninstall - reboot - remove preferences - reboot - reinstall - reboot is letting me build again.

Of course. In the meantime, I shall do my uttermost to break it again

FWIW and not that I have either the time or inclination to investigate, the stack trace is as follows:

2006-11-23 14:05:26.305 Xcode[7787] ** INTERNAL ERROR: Uncaught Exception **
Exception: *** -[NSCFDictionary setObject:forKey:]: attempt to insert nil key
Stack:
0 0x956c1b90 -[TSException raise]
1 0x9297be58 +[NSException raise:format:]
2 0x92942464 -[NSCFDictionary setObject:forKey:]
3 0x99279814 -[PBXAEEditorSpecification _modifiedFileDictionary]
4 0x99279618 -[PBXAEEditorSpecification modifiedDocumentsArray]
5 0x991106bc -[PBXExternalEditorInterface modifiedDocumentsFromExternalEditors]
6 0x99110428 -[PBXProjectDocument editedFileDocuments]
7 0x99110240 -[PBXProjectDocument saveEditedDocumentsIfNeededWithCancelTitle:]
8 0x99331ee0 -[XCProjectSelection buildTargets:andDependencies:withAction:usingBuildStyles:usingBuildConfigurationverridingBuildSettings:notifyingObject:]
9 0x991101e4 -[XCProjectSelection buildTargets:withAction:]
10 0x90a441f0 _objc_msgSendv
11 0x9295cc84 -[NSInvocation invoke]
12 0x9295d234 -[NSInvocation invokeWithTarget:]
13 0x92955030 -[NSObject(NSForwardInvocation) forward::]
14 0x90a440ac __objc_msgForward
15 0x937acc48 -[NSApplication sendAction:to:from:]
16 0x938074b4 -[NSMenu performActionForItemAtIndex:]
17 0x93807238 -[NSCarbonMenuImpl performActionWithHighlightingForItemAtIndex:]
18 0x93806ce0 -[NSMenu performKeyEquivalent:]
19 0x938068c0 -[NSApplication _handleKeyEquivalent:]
20 0x9908ba1c -[PBXExtendedApplication sendEvent:]
21 0x93707d0c -[NSApplication run]
22 0x937f8878 _NSApplicationMain
23 0x00009b04
24 0x00009808
All identities removed.

but it may be a little more meaningful to someone with a reasonable knowledge of Cocoa event handling.