[syncfusion_flutter_charts] crash with null xAxis

[mockturtl]

Bug description

From versions 30.1.39 (possibly earlier) to 30.2.4 (latest), we see set xAxis(RenderChartAxis? value) (axis.dart) receiving null, which violates an assertion in the setter.

The following assertion was thrown while rebuilding dirty elements:
'package:syncfusion_flutter_charts/src/charts/axis/axis.dart': Failed assertion: line 3855 pos 12: 'value != null': is not true.

https://github.com/syncfusion/flutter-widgets/blame/master/packages/syncfusion_flutter_charts/lib/src/charts/axis/axis.dart#L3855

• When we comment out the failing assertion, the stack shows a few more places where the ! operator is incorrectly applied to a null value.

The following _TypeError was thrown during paint():
Null check operator used on a null value

The relevant error-causing widget was:  [REDACTED]
When the exception was thrown, this was the stack: 
#0      LineSeriesRenderer.onPaint (package:syncfusion_flutter_charts/src/charts/series/line_series.dart:148:24)

https://github.com/syncfusion/flutter-widgets/blame/master/packages/syncfusion_flutter_charts/lib/src/charts/series/line_series.dart#L148

https://github.com/syncfusion/flutter-widgets/blame/master/packages/syncfusion_flutter_charts/lib/src/charts/series/chart_series.dart#L5590

• Simply replacing the ! with ? seems to resolve our issue.

I believe this is a bug in the library -- since the type RenderChartAxis? allows null, it should be handled by all callers which read the xAxis property.

Please let me know if you'd like me to submit a PR which fixes the issue for us.

Steps to reproduce

Unfortunately, I cannot provide steps to reproduce.

Code sample

Unfortunately, I cannot provide a minimal code sample.

Screenshots or Video

(no screenshots/video)

Stack Traces

(see above)

On which target platforms have you observed this bug?

Windows

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