Fix crash when setting custom VM icon (#2387)

[ktprograms]

It turns out there were two unrelated problems.

First, and caused the crash (Line 199):
The first time you set a custom icon, the self.configuration.icon is nil, which crashes NSURL's - (nullable NSURL *)URLByAppendingPathComponent:(NSString *)pathComponent. I changed the code to check for self.configuration.icon being nil so it would skip over the deleting the previous icon.

Second, fix custom icon not being set creating a new VM (Line 143):
The setting self.configuration with the configuration parameter wasn't setting the selectedCustomIconPath. I added an extra line to also set just the selectedCustomIconPath.