Crash when emptying a string in git flow

SourceTree 1.9.2 crashed (or at least threw an assertion) when working in git flow and deleting the release version name.

Steps to reproduce:

• launch SourceTree
• open a repo (a git repo in this case)
• click the "git flow" toolbar icon
• click "start a new release" (this is what I was doing)
• start typing something in the "release version" text field. In my case, I started to type "1", then realized I wanted to name it something else.
• press the delete/backspace key to delete that 1 character.
• internal error

Error log:

Exception Name: NSInternalInconsistencyException
Description: Invalid parameter not satisfying: aString != nil
User Info: (null)

0 CoreFoundation 0x00007fff8e70f25c __exceptionPreprocess + 172
1 libobjc.A.dylib 0x00007fff8dc38e75 objc_exception_throw + 43
2 CoreFoundation 0x00007fff8e70f038 +[NSException raise:format:arguments:] + 104
3 Foundation 0x00007fff9586ad41 -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 189
4 AppKit 0x00007fff929598d8 -[NSCell _objectValue:forString:errorDescription:] + 159
5 AppKit 0x00007fff9295982a -[NSCell _objectValue:forString:] + 20
6 AppKit 0x00007fff92959798 -[NSCell setStringValue:] + 41
7 AppKit 0x00007fff929d55b4 -[NSControl setStringValue:] + 138
8 CoreFoundation 0x00007fff8e6dde0c _CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER_ + 12
9 CoreFoundation 0x00007fff8e5d18dd _CFXNotificationPost + 2893
10 Foundation 0x00007fff957757ba -[NSNotificationCenter postNotificationName:object:userInfo:] + 68
11 CoreFoundation 0x00007fff8e6dde0c _CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER_ + 12
12 CoreFoundation 0x00007fff8e5d18dd _CFXNotificationPost + 2893
13 Foundation 0x00007fff957757ba -[NSNotificationCenter postNotificationName:object:userInfo:] + 68
14 AppKit 0x00007fff92b771b4 -[NSTextView(NSSharing) didChangeText] + 345
15 AppKit 0x00007fff92bcc2a1 _NSDoUserReplaceForCharRange + 466
16 AppKit 0x00007fff92bcc0c2 _NSDoUserDeleteForCharRange + 38
17 AppKit 0x00007fff92bcb882 -[NSTextView(NSKeyBindingCommands) deleteBackward:] + 841
18 AppKit 0x00007fff92b51c2f -[NSResponder doCommandBySelector:] + 71
19 AppKit 0x00007fff92b7f10a -[NSTextView doCommandBySelector:] + 196
20 AppKit 0x00007fff92b51151 -[NSKeyBindingManager(NSKeyBindingManager_MultiClients) interpretEventAsCommand:forClient:] + 1392
21 AppKit 0x00007fff92b701c2 -[NSTextInputContext handleEvent:] + 845
22 AppKit 0x00007fff92b4f9dd -[NSView interpretKeyEvents:] + 180
23 AppKit 0x00007fff92b6fd6d -[NSTextView keyDown:] + 658
24 AppKit 0x00007fff92b1c56b -[NSWindow sendEvent:] + 1843
25 AppKit 0x00007fff92abdb32 -[NSApplication sendEvent:] + 3395
26 AppKit 0x00007fff9290da19 -[NSApplication run] + 646
27 AppKit 0x00007fff928f87a3 NSApplicationMain + 940
28 SourceTree 0x00000001065d6254 SourceTree + 8788