Setting the path to toolbarAssociatedXibName, with fallback
In the spirit of DRY, how about:

NSString *path = nil;
if (delegate &amp;&amp; [delegate respondsToSelector:@selector(toolbarAssociatedXibName:)]) {
    path = [delegate toolbarAssociatedXibName:_toolbar];
    if (![[NSBundle mainBundle] pathForResource:path ofType:@"nib"]) {
        IFLog(@"Delegate responded with an invalid value (%@) for `toolbarAssociatedXibName:`. Attempting default value (%@).", path, _identifier);
        path = nil;
    }
}
if (!path) path = _identifier;


In your pseudocode:

Variable declaration (set to null value)
If there is an option to set the variable to a custom value
    Set it to that custom value
    If the value is not valid
        Log the error
        Set the value back to null
If value is null
    Set the value to a specified default