Clicking `pause` button can crash DevTools

[elliette]

Ran into this debugging an internal app. Because we are null-asserting mainIsolate when checking whether the app is paused, if the mainIsolate is null DevTools will crash:

devtools/packages/devtools_app/lib/src/screens/debugger/controls.dart

Line 37 in a235825

serviceManager.isolateManager.mainIsolateState!.isPaused,

Stack trace from Chrome DevTools:

main.dart.js:12053 Uncaught 
main.dart.js:75177 NoSuchMethodError: method not found: '_isPaused' on null
log @ main.dart.js:75177
main.dart.js:74215 NoSuchMethodError: method not found: '_isPaused' on null
main.dart.js:74215     at _DebuggingControlsState.didChangeDependencies$0 (https://b2607f8b04800100000144d90c0a823628fa9000000000000000001.proxy.googlers.com/main.dart.js:119632:168)
main.dart.js:74215     at StatefulElement._firstBuild$0 (https://b2607f8b04800100000144d90c0a823628fa9000000000000000001.proxy.googlers.com/main.dart.js:252207:30)
main.dart.js:74215     at StatefulElement.mount$2 (https://b2607f8b04800100000144d90c0a823628fa9000000000000000001.proxy.googlers.com/main.dart.js:252139:12)
main.dart.js:74215     at MultiChildRenderObjectElement.inflateWidget$2 (https://b2607f8b04800100000144d90c0a823628fa9000000000000000001.proxy.googlers.com/main.dart.js:251885:19)

[srawlins]

Closing as resolved: isMainIsolatePaused and isolateManager.mainIsolateState are now safely handled with null-safe access throughout debugging controls and service management, avoiding null assertions when mainIsolateState is null.

If there is more work to do here, please let us know by filing a new issue with up to date information. Thanks!