Unable to clear polylines set for map (worked previously)

[fbazbo]

Describe the bug
Calling the clear method on map.polylines throws "Unhandled Exception: Unsupported operation: Cannot change an unmodifiable set"

To Reproduce
Steps to reproduce the behavior:

1. Just add a clear() call somewhere, though may need to add a few polylines if you don't have them in the sample.

Expected behavior
the set should be mutable and the clear should remove all displayed polylines.

Screenshots
n/a.

Smartphone (please complete the following information):

• Device: Iphone simulator (any)
• Version [e.g. 22]

flutter doctor
/Volumes/devstuff/Developer/flutter/bin/flutter doctor --verbose
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• Framework revision ffb2ecea52 (7 weeks ago), 2021-09-17 15:26:33 -0400
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[✓] Connected device (3 available)
• iPhone 8 (mobile) • B094B1C2-EBDB-404C-A35A-58F6931095AC • ios • com.apple.CoreSimulator.SimRuntime.iOS-15-0 (simulator)
• iPhone 13 Pro (mobile) • 38883115-5DA6-4341-A3DE-5C06313A80B0 • ios • com.apple.CoreSimulator.SimRuntime.iOS-15-0 (simulator)
• Chrome (web) • chrome • web-javascript • Google Chrome 95.0.4638.69

• No issues found!

[fbazbo]

I added a check of a length > 0 before calling clear and it seems to have fixed the issue, so it must be a change in behavior when the list is empty