Flutter: "(path): must not be null" on conditional file checks - image

Question

I am trying to have a background to my application that either the user has chosen previously, or to use a default image if no file is found.
The code is very simple:
decoration: BoxDecoration(
color: Colors.blueAccent,
image: DecorationImage(
image: File(customImageFile).existsSync() ? FileImage(File(customImageFile)) : backgroundImage,
fit: BoxFit.fill,
),
),
The application crashes on the conditional line, saying that (path): Must not be null. I have tried many ways to get around this, but what I am specifically saying there is "if the file path is null, don't use it. Use this image I made earlier for you". But then it gets upset anyway.
Is there another way I can achieve the intended result here?
Edit:
As requested by #Vikas, more complete sample of code involved:
ImageProvider backgroundImage;
String themeLoader = 'blueTheme';
String customImageFile;
// Declaration
void initState() {
_loadThemeSettings();
super.initState();
}
// Initialisation
_loadThemeSettings() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
themeLoader = prefs.getString('themeColour' ?? 'blueTheme');
customImageFile = prefs.getString('customImageFile' ?? 'empty')
if (customImageFile != null) {
if(File(customImageFile).existsSync()) {
prefs.setString('customImageFile', customImageFile);
} else {
print('Path not valid');
}
} else {
print('customImageFile is NULL');
}
if (themeLoader != null) {
switch (themeLoader) {
case 'blueTheme':
{
colorBloc.changeColor('blueTheme');
setState(() {
backgroundImage = ExactAssetImage('lib/images/bg_blue_new.jpg');
buttonTheme = AppState.blueTheme;
}
}
break;
} else {
print('Theme loader was NULL');
}
}
}
}
// Loading previously selected themes at run time. There are several colours here in the case statement, but they are all identical and you get the idea.
#override
Widget build(BuildContext context) {
return WillPopScope(
onWillPop: () => _exitApp(context),
child: Scaffold(
body: SafeArea(
child: BlocProvider<ColorBloc>(
bloc: colorBloc,
child: Container(
child: Stack(
children: [
Container(
decoration: BoxDecoration(
color: Colors.blueAccent,
image: DecorationImage(
image: (customImageFile != null && File(customImageFile).existsSync()) ? FileImage(File(customImageFile)) : backgroundImage,
fit: BoxFit.fill,
),
),
// that's where it sits, everything else works fine. The rest of the build method is not relevant to this issue, and is working, but here it is anyway:
child: SafeArea(
child: Stack(
children: <Widget>[
Column(
mainAxisAlignment: MainAxisAlignment.end,
children: <Widget>[
themesVisible == true
? SizedBox()
: buttonRow(),
SizedBox(
height: 20,
),
],
),
Padding(
padding: EdgeInsets.all(15),
child: topMenuRow(),
),
],
),
),
),
BlocProvider<CustomToggleBloc>(
bloc: customToggleBloc,
child: Center(
child: Container(
child: themesVisible == true ? singlePlayerThemes(singlePlayerCallbacks: callBacks) : scoreText(),
),
),
),//bloc
],
),
),
),
),
),
);
}