Error map
FormatException: Unexpected character Dart: FormatException: Unexpected character (at character 1) <!DOCTYPE html> — jsonDecode of an HTML page
Flutter / Dart · layer 1, On your device
Centre: this error. First ring: errors it is linked with (11). Outer ring: what those are linked with. Colours show the layer of a request each error belongs to. Click any dot to re-centre the map on it; hover to follow its connections. The centre dot opens the error page.
Directly related (11)
Dart: The argument type 'String?' can't be assigned to the parameter type 'String'
You passed a value whose type does not match what the function parameter requires.
Flutter: Unhandled Exception: SocketException: Failed host lookup: 'api.example.com' (OS Error: No address associated with hostname, errno = 7)
The app could not resolve or reach the server: no internet, wrong host name, DNS failure, or missing network permission.
Dart: type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'List<dynamic>' in type cast
JSON decoded to a Map but your code expected a List (or the other way round).
Flutter (Android): java.io.IOException: Cleartext HTTP traffic to api.example.com not permitted
Android 9+ blocks plain `http://` requests by default; the app must use HTTPS or explicitly allow cleartext for that host.
Dart: Bad state: No element — .first, .last or .single on an empty list
You called `.first`, `.last`, `.single` or `.reduce` on an empty collection.
Dart: type 'Null' is not a subtype of type 'String' in type cast
A value that was `null` at runtime was cast or used as a non-nullable type, most often when parsing JSON.
Dart: RangeError (index): Invalid value: Valid value range is empty: 0
A list or string was indexed outside its bounds, often an empty list or an off-by-one in `itemBuilder`.
Dart: Bad state: Stream has already been listened to
A single-subscription stream was listened to twice, for instance by two `StreamBuilder`s or after a rebuild.
Dart: Error: Couldn't resolve the package 'x' in 'package:x/x.dart'
Code imports a package that is not in `pubspec.yaml` or `pub get` has not been run since adding it.
Dart: The getter 'name' isn't defined for the type 'User' / The method 'foo' isn't defined for the class
The analyzer cannot find that property or method on the object's static type.
Dart: Invalid argument(s): Illegal argument in isolate message: object is unsendable - Library:'dart:async' Class: _Future@4048458
You tried to send an object (closure, Future, socket, platform object) to another isolate that cannot be copied.
Two steps away (17)
Swift: Cannot convert value of type 'X' to expected argument type 'Y'
The value’s type doesn’t match what the function or property expects.
Dart/Flutter: Null check operator used on a null value
The ! operator was applied to a variable that was null at runtime.
Dart: LateInitializationError: Field 'controller' has not been initialized
A `late` variable was read before it was assigned.
Flutter: BoxConstraints forces an infinite width. These invalid constraints were provided to RenderStack's layout() function (TextField in a Row)
A widget that needs a finite width, like a `TextField` or `Container` with no size, was placed where the parent offers unlimited…
Dart: HandshakeException: Handshake error in client (OS Error: CERTIFICATE_VERIFY_FAILED: unable to get local issuer certificate)
The app refused the server's TLS certificate because it is self-signed, expired, missing intermediates, or the device clock/CA st…
Flutter: Exception caught by image resource service — NetworkImageLoadException: HTTP request failed, statusCode: 404, https://example.com/a.png
`Image.network` could not download the image (bad URL, status error, or blocked by CORS on web).
android.os.NetworkOnMainThreadException
The app made a network call on the main (UI) thread, which Android forbids.
Error Domain=NSURLErrorDomain Code=-1009 "The Internet connection appears to be offline."
URLSession couldn’t reach the network — the device is offline, the app’s cellular data is disabled, or the simulator lost network…
Error Domain=NSURLErrorDomain Code=-1001 "The request timed out."
The server didn’t respond within URLSession’s timeout (60 s default) — slow server, unreachable host on this network, or a stalle…
Dart: Non-nullable instance field 'name' must be initialized. Try adding an initializer expression, or a generative constructor that initializes it, or mark it 'late'
With null safety every non-nullable field needs a value by the end of construction.
Dart: The property 'length' can't be unconditionally accessed because the receiver can be 'null'
With null safety you used `.` on a nullable value; Dart requires a null check first.
Dart: The named parameter 'key' is required, but there's no corresponding argument
A constructor or function marked `required` for a parameter was called without it.
Flutter: setState() or markNeedsBuild() called during build. This widget cannot be marked as needing to build because the framework is already in the process of building widgets
State was changed while Flutter was building widgets, for example calling `setState` or a notifier from inside `build`.
Flutter: _HomeState#abc(ticker active) was disposed with an active Ticker. State.dispose() implementations must call dispose() on all their AnimationControllers
An `AnimationController` was still running when its `State` was disposed, because `dispose()` did not release it.
Dart pub: Because app depends on intl ^0.19.0 and flutter_localizations from sdk depends on intl 0.18.1, version solving failed
`flutter pub get` cannot find a set of package versions that satisfy every dependency's constraints.
Dart pub: The current Dart SDK version is 2.19.0. Because app requires SDK version >=3.0.0 <4.0.0, version solving failed
A package (or your own pubspec) requires a newer Dart SDK than the one your Flutter installation provides.
Flutter: Looking up a deactivated widget's ancestor is unsafe. At this point the state of the widget's element tree is no longer stable
You used a `BuildContext` after its widget was removed from the tree — typically after an `await` when the user already navigated…
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