RichTextEditor: Ensure prevLine is defined before subsequently calling .split on it

[rosslavery]

Ensures that prevLine is defined before calling split on it, as well as any other function calls that follow.

Prevents the following error:

core.js:15723 ERROR TypeError: Cannot read property 'split' of undefined
    at MDLists.push../node_modules/@syncfusion/ej2-richtexteditor/src/markdown-parser/plugin/lists.js.MDLists.enterKey (lists.js:259)
    at MDLists.push../node_modules/@syncfusion/ej2-richtexteditor/src/markdown-parser/plugin/lists.js.MDLists.keyUpHandler (lists.js:44)
    at Observer.push../node_modules/@syncfusion/ej2-base/src/observer.js.Observer.notify (observer.js:99)
    at MarkdownParser.push../node_modules/@syncfusion/ej2-richtexteditor/src/markdown-parser/base/markdown-parser.js.MarkdownParser.editorKeyUp (markdown-parser.js:54)
    at Observer.push../node_modules/@syncfusion/ej2-base/src/observer.js.Observer.notify (observer.js:99)
    at MarkdownFormatter.push../node_modules/@syncfusion/ej2-richtexteditor/src/rich-text-editor/formatter/formatter.js.Formatter.onKeyHandler (formatter.js:102)
    at RichTextEditorComponent.push../node_modules/@syncfusion/ej2-richtexteditor/src/rich-text-editor/base/rich-text-editor.js.RichTextEditor.keyUp (rich-text-editor.js:395)
    at HTMLTextAreaElement.__trace__ (bugsnag.js:2211)
    at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invokeTask (zone.js:421)
    at Object.onInvokeTask (core.js:17289)

[pandiyarajm]

Hi @rosslavery ,

Greetings from Syncfusion support.

We have tried to reproduce the issue that you have done the source changes, but we were not able to reproduce the issue at our end. Could you please share the details which scenario cause issue, without this changes that will be helpful for us to get the actual issue?

1. Share the example code block
2. Share the stack trace if any script error is thrown

Regards,
Pandiyaraj M

[Sridhar-Narasimhan]

We are closing this thread now since there is no activity and long pending.