The dialog says:
SourceTree detected the following invalid Dictionary file references. These are known to cause performance issues.
Do you want to remove these invalid dictionaries?
I click "Yes" every time, and it still comes back the next time I start up.
As mentioned below, if you look in the sourcetree.log file you can find out which dictionary file is causing the message. In my case I had a dictionary file in my roaming profile that was causing it. You may have to check the log file multiple times until the message is not displayed anymore.
In my case SourceTree had a "stuck" dictionary file. I had to delete it manually.
This is on Windows 10.
You'll need to first find the location of the file.
Open SourceTree's log file in <user directory>\AppData\Local\Atlassian\SourceTree\sourcetree.log
Look for where it says something like this. Most recent entries are at the bottom.
ERROR [2016-12-16 11:24:35,973]  [SourceTree.Configuration.WpfSpellCheckerPreFlightCheck] [Run] - Keeping dictionary reference [C:\Users\<user>\AppData\Local\Temp\u3vdz0px.dic] ERROR [2016-12-16 11:24:36,524]  [SourceTree.Configuration.WpfSpellCheckerPreFlightCheck] [Run] - Removing non-existent dictionary reference 
Note the location of the .dic and delete it.
THIS one is the right answer.
In my case I deleted ALL the Temp folder content.
Next start of SourceTree showed the error but this time the list of disctionaires were there.
I clicked "Yes" (to remove the dictionaries), closed and restarted.
Error is no more there.
I also deleted the SourceTree_Url_ghasdhkfdsahkfg folder near the "SourceTree" one.
It contains a folder for every version you have installed (also the current one).
Bad idea. On the successive start of the program it begun the install procedure. I stopped it and restored the deleted folder (from the trash bin). Just 2 files about "configuration" (sorry, I haven't noted the names) was changed. I restored all with the old one.
Start again: OK.
I added this details to share the real experience I had but I think the solution was just to remove the bad disctionaries from the "Temp" folder.
Many thanks Isaac.
I no longer get a dialog, but the log still shows this un-error: ERROR [2016-12-22 10:59:52,113]  [SourceTree.Configuration.WpfSpellCheckerPreFlightCheck] [Run] - Keeping dictionary reference [C:\Users\Sonny\AppData\Roaming\Microsoft\Office\16.0\5602c0c7\Proofing\RoamingCustom.dic]
In my log file, I see a message about "Removing non-existent dictionary reference ". It seems to be an empty string or something. I can't find anything to delete.
ERROR [2017-01-01 04:43:26,708]  [SourceTree.Configuration.WpfSpellCheckerPreFlightCheck] [Run] - Removing non-existent dictionary reference 
I also have this issue and this solution hasn't worked for me. Currently running version 188.8.131.52. If I disable spell checking then there's no error but that's not a suitable solution for me.
I've tried repairing the installation and uninstall/reinstall but the issue still persists.
Have you found and deleted the missing dictionary from the Temp folder?
If this did not worked and you don't care to "rollback" to a previous version you can try to delete the
old settings of previous version of SourceTree.
In <user directory>\AppData\Local\Atlassian\SourceTree.exe_Url_yjhzxxhcu2ommegrdorjf3q1oqllqxiq
you can delete the directories of the old versions of SourceTree and leave only the current one (184.108.40.206).
On a different Windows 10 I had the same issues (solved on the other PC) and this was the simplest solution (I was too lazy to check the log and the precise dictionary files and I prefer to clean this old stuff).
I had to delete all the .dic files in following locations to get my machine to stop complaining:
Thanks for the tip and all the comments.
Solved the empty list complaint!!
In the registry value:
Computer\HKEY_CURRENT_USER\SOFTWARE\Microsoft\Spelling\Dictionaries in the property named "_Global_", there is a list of dictionary names. Right-click-> Modify on the value and the list of entries pops up in an edit window. Scroll down to the bottom, of the list there was a blank row with a carriage return, and I removed that row and it then stopped giving me the error where it complained when I launched it.
It was weird that after removing the blank line and I clicked OK, it popped up a message that said "Data of type REG_MULTI_SZ cannot contain empty strings. Registry Editor will remove all empty strings." I clicked OK on that message and everything is now fixed.
Modify to get this widow, then remove blank line at bottom of list:
When I clicked OK, it gave this warning:
But now the problem is FIXED!!
I had to delete dictionary file ( "dictionary.lex" ) which was is located at C:\Users\<username>\AppData\Local\Atlassian\SourceTree folder and restart SourceTree. Its working fine now.
Once the application started it recreated "dictionary.lex". The error is gone and spell check is working as it should.
I'm in windows 10.
I searched for .dic in the
folder, then deleted all. After restarting SourceTree I received the same invalid dictionary message however this time there was actually a list of invalid .dic files in the message. I confirmed yes to removing them and then restarted again.
Error message gone.
After deleting all *.dic files from the temp folder, I still get the folllowing error as @Edward Savage:
ERROR [2016-12-29 18:39:48,560]  [SourceTree.Configuration.WpfSpellCheckerPreFlightCheck] [Run] - Keeping dictionary reference [C:\Users\bibi\AppData\Roaming\Microsoft\Office\15.0\bbae5a57\Proofing\RoamingCustom.dic]
The beginning of the file(RoamingCustom.dic) looks like this:
' 'Anita 'Daniel 'David 'Denis
I deleted the first line (the one with the single "single quote") and the error went away
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