Update README.md

When leaving the Code Editor of certain Extensions, this Exception would throw:
```
[ERROR:flutter/runtime/dart_vm_initializer.cc(40)] Unhandled Exception: Runtime Error: Undefined variable: source (Original error: Undefined property 'source' on Madara.)
#0      InterpreterVisitor.visitSimpleIdentifier (package:d4rt/src/interpreter_visitor.dart:393:7)
interpreter_visitor.dart:393
#1      SimpleIdentifierImpl.accept (package:analyzer/src/dart/ast/ast.dart:16123:50)
ast.dart:16123
#2      InterpreterVisitor.visitPostfixExpression (package:d4rt/src/interpreter_visitor.dart:3930:41)
interpreter_visitor.dart:3930
#3      PostfixExpressionImpl.accept (package:analyzer/src/dart/ast/ast.dart:14311:50)
ast.dart:14311
#4      InterpreterVisitor._evaluateArguments (package:d4rt/src/interpreter_visitor.dart:5569:23)
interpreter_visitor.dart:5569
#5      InterpreterVisitor.visitMethodInvocation (package:d4rt/src/interpreter_visitor.dart:2178:13)
interpreter_visitor.dart:2178
kodjodevf#6      MethodInvocationImpl.accept (package:analyzer/src/dart/ast/ast.dart:12246:50)
ast.dart:12246
kodjodevf#7      InterpretedClass.createAndInitializeInstance (package:d4rt/src/runtime_types.dart:232:53)
runtime_types.dart:232
kodjodevf#8      InterpretedClass.call (package:d4rt/src/runtime_types.dart:299:22)
runtime_types.dart:299
kodjodevf#9      InterpreterVisitor.visitMethodInvocation (package:d4rt/src/interpreter_visitor.dart:2161:28)
interpreter_visitor.dart:2161
kodjodevf#10     MethodInvocationImpl.accept (package:analyzer/src/dart/ast/ast.dart:12246:50)
ast.dart:12246
kodjodevf#11     InterpreterVisitor.visitVariableDeclarationList (package:d4rt/src/interpreter_visitor.dart:3026:42)
interpreter_visitor.dart:3026
kodjodevf#12     VariableDeclarationListImpl.accept (package:analyzer/src/dart/ast/ast.dart:18695:15)
ast.dart:18695
kodjodevf#13     InterpreterVisitor.visitVariableDeclarationStatement (package:d4rt/src/interpreter_visitor.dart:135:27)
interpreter_visitor.dart:135
kodjodevf#14     VariableDeclarationStatementImpl.accept (package:analyzer/src/dart/ast/ast.dart:18753:15)
ast.dart:18753
kodjodevf#15     InterpreterVisitor.executeBlock (package:d4rt/src/interpreter_visitor.dart:66:33)
interpreter_visitor.dart:66
kodjodevf#16     InterpretedFunction.call (package:d4rt/src/callable.dart:589:36)
callable.dart:589
kodjodevf#17     D4rt.execute (package:d4rt/src/d4rt_base.dart:220:43)
d4rt_base.dart:220
kodjodevf#18     DartExtensionService._executeLib (package:mangayomi/eval/dart/service.dart:24:17)
service.dart:24
kodjodevf#19     DartExtensionService.getSourcePreferences (package:mangayomi/eval/dart/service.dart:179:25)
service.dart:179
kodjodevf#20     getSourcePreference (package:mangayomi/services/get_source_preference.dart:6:38)
get_source_preference.dart:6
kodjodevf#21     _ExtensionDetailState.build.<anonymous closure>.<anonymous closure> (package:mangayomi/modules/browse/extension/extension_detail.dart:173:44)
extension_detail.dart:173
kodjodevf#22     State.setState (package:flutter/src/widgets/framework.dart:1199:30)
framework.dart:1199
kodjodevf#23     _ExtensionDetailState.build.<anonymous closure> (package:mangayomi/modules/browse/extension/extension_detail.dart:171:23)
extension_detail.dart:171
<asynchronous suspension>
```