NoSuchMethodError: _layout! null check in build() when LayoutBuilder receives zero-height constraints

[chan-vince]

On Flutter web, PdfViewer crashes with NoSuchMethodError: Null check operator used on a null value at pdf_viewer.dart:525 (_layout!.documentSize) when its LayoutBuilder fires with maxHeight <= 0 — e.g. during a route transition that briefly collapses the parent's height.

_updateLayout early-returns when viewSize.height <= 0 (per its comment, this exists to fix a blank PDF when restoring a window from minimize on Windows) and leaves _layout null, but build() then force-unwraps it on the same frame.

The text-selection branch (:547 in 2.2.24, :674 in 2.4.1) has the same _layout! unwrap.

Stack:

NoSuchMethodError: Null check operator used on a null value
  at _PdfViewerState.build.<anonymous function>.<anonymous function>
     (pdfrx-2.2.24/lib/src/widgets/pdf_viewer.dart:525:48)

[espresso3389]

Fixed in a37bc57d.

What changed:

• After _updateLayout(viewSize), PdfViewer now checks whether _layout is still null.
• If the current frame has no usable layout yet, for example because LayoutBuilder temporarily received zero-height constraints, build() returns an empty widget for that frame instead of continuing to _layout!.
• This keeps the existing _updateLayout() early return for viewSize.height <= 0, which was added for the Windows restore-from-minimize case, while avoiding the null-check crash reported here.

Relevant files:

• pdf_viewer.dart
• pdf_viewer_test.dart

I also added a regression test that starts PdfViewer inside a zero-height parent, verifies no exception is thrown, then expands the parent and verifies the viewer initializes successfully.

Verification run:

dart format lib/src/widgets/pdf_viewer.dart test/pdf_viewer_test.dart
flutter analyze lib/src/widgets/pdf_viewer.dart test/pdf_viewer_test.dart
flutter test test/pdf_viewer_test.dart

All passed.