Prevent code action from exception

[angelozerr]

Given this XML:

<a>

  </

</a>

When #880 will be implemented, when we hover to root element a, we should see several code action, but there are no code actions:

We have a the following stack traces:

[Error - 12:25:10] Request textDocument/codeAction failed.
  Message: Internal error.
  Code: -32603 
java.util.concurrent.CompletionException: java.lang.NullPointerException
	at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:314)
	at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:319)
	at java.base/java.util.concurrent.CompletableFuture.biApply(CompletableFuture.java:1238)
	at java.base/java.util.concurrent.CompletableFuture$BiApply.tryFire(CompletableFuture.java:1205)
	at java.base/java.util.concurrent.CompletableFuture$Completion.exec(CompletableFuture.java:479)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:290)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1020)
	at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1656)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1594)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:177)
Caused by: java.lang.NullPointerException
	at java.base/java.lang.StringBuilder.<init>(StringBuilder.java:124)
	at org.eclipse.lemminx.utils.XMLBuilder.trimTrailingSpacesEachLine(XMLBuilder.java:374)
	at org.eclipse.lemminx.utils.XMLBuilder.addContent(XMLBuilder.java:303)
	at org.eclipse.lemminx.utils.XMLBuilder.addContent(XMLBuilder.java:277)
	at org.eclipse.lemminx.extensions.generators.xml2dtd.XML2DTDGenerator.generate(XML2DTDGenerator.java:96)
	at org.eclipse.lemminx.extensions.generators.xml2dtd.XML2DTDGenerator.generate(XML2DTDGenerator.java:29)
	at org.eclipse.lemminx.extensions.generators.AbstractXML2GrammarGenerator.doGenerate(AbstractXML2GrammarGenerator.java:67)
	at org.eclipse.lemminx.extensions.generators.AbstractXML2GrammarGenerator.generate(AbstractXML2GrammarGenerator.java:47)
	at org.eclipse.lemminx.extensions.generators.AbstractXML2GrammarGenerator.generate(AbstractXML2GrammarGenerator.java:40)
	at org.eclipse.lemminx.extensions.generators.FileContentGeneratorManager.generate(FileContentGeneratorManager.java:88)
	at org.eclipse.lemminx.extensions.contentmodel.participants.codeactions.NoGrammarConstraintsCodeAction.doCodeAction(NoGrammarConstraintsCodeAction.java:93)
	at org.eclipse.lemminx.extensions.contentmodel.participants.ContentModelCodeActionParticipant.doCodeAction(ContentModelCodeActionParticipant.java:49)
	at org.eclipse.lemminx.services.XMLCodeActions.doCodeActions(XMLCodeActions.java:45)
	at org.eclipse.lemminx.services.XMLLanguageService.doCodeActions(XMLLanguageService.java:255)
	at org.eclipse.lemminx.XMLTextDocumentService.lambda$codeAction$19(XMLTextDocumentService.java:389)
	at java.base/java.util.concurrent.CompletableFuture.biApply(CompletableFuture.java:1236)
	... 7 more

This issue comes from the </ which have a null local name.

To fix correctly this issue, we should:

• try catch each code action to display code action which doesn't throw exception.
• fix the DTD generator to take care of element which have null tag name.

[angelozerr]

Fixed with #897