postfixCompletion proposal computer did not complete normally

When having "Java Postfix Template Proposals" enabled, with the following code
```java
class MyClass {
MyClass(Object o) {
}
}
```
and the caret between `(` and `Object` of the `MyClass` constructor, when activating Content Assist the following exception is thrown:

> The 'org.eclipse.jdt.ui.postfixCompletion' proposal computer from the 'org.eclipse.jdt.ui' plug-in did not complete normally. The extension has thrown a runtime exception.

```stacktrace
java.lang.IllegalArgumentException:
at org.eclipse.jdt.core.Signature.createCharArrayTypeSignature(Signature.java:1105)
at org.eclipse.jdt.core.Signature.createTypeSignature(Signature.java:1305)
at org.eclipse.jdt.internal.core.CompilationUnitStructureRequestor.convertTypeNamesToSigs(CompilationUnitStructureRequestor.java:292)
at org.eclipse.jdt.internal.codeassist.CompletionUnitStructureRequestor.createMethodHandle(CompletionUnitStructureRequestor.java:143)
at org.eclipse.jdt.internal.core.CompilationUnitStructureRequestor.enterMethod(CompilationUnitStructureRequestor.java:419)
at org.eclipse.jdt.internal.core.CompilationUnitStructureRequestor.enterConstructor(CompilationUnitStructureRequestor.java:345)
at org.eclipse.jdt.internal.compiler.SourceElementNotifier.notifySourceElementRequestor(SourceElementNotifier.java:334)
at org.eclipse.jdt.internal.codeassist.CompletionElementNotifier.notifySourceElementRequestor(CompletionElementNotifier.java:197)
at org.eclipse.jdt.internal.compiler.SourceElementNotifier.notifySourceElementRequestor(SourceElementNotifier.java:804)
at org.eclipse.jdt.internal.codeassist.CompletionElementNotifier.notifySourceElementRequestor(CompletionElementNotifier.java:226)
at org.eclipse.jdt.internal.compiler.SourceElementNotifier.notifySourceElementRequestor(SourceElementNotifier.java:488)
at org.eclipse.jdt.internal.codeassist.CompletionElementNotifier.notifySourceElementRequestor(CompletionElementNotifier.java:202)
at org.eclipse.jdt.internal.codeassist.InternalExtendedCompletionContext.computeEnclosingJavaElements(InternalExtendedCompletionContext.java:162)
at org.eclipse.jdt.internal.codeassist.InternalExtendedCompletionContext.getEnclosingElement(InternalExtendedCompletionContext.java:255)
at org.eclipse.jdt.internal.codeassist.InternalCompletionContext.getEnclosingElement(InternalCompletionContext.java:128)
at org.eclipse.jdt.internal.ui.text.java.PostfixCompletionProposalComputer.updateTemplateEngine(PostfixCompletionProposalComputer.java:142)
at org.eclipse.jdt.internal.ui.text.java.PostfixCompletionProposalComputer.analyzeCoreContext(PostfixCompletionProposalComputer.java:121)
at org.eclipse.jdt.internal.ui.text.java.PostfixCompletionProposalComputer.computeCompletionEngine(PostfixCompletionProposalComputer.java:110)
at org.eclipse.jdt.internal.ui.text.java.AbstractTemplateCompletionProposalComputer.computeCompletionProposals(AbstractTemplateCompletionProposalComputer.java:71)
at org.eclipse.jdt.internal.ui.text.java.CompletionProposalComputerDescriptor.computeCompletionProposals(CompletionProposalComputerDescriptor.java:348)
at org.eclipse.jdt.internal.ui.text.java.CompletionProposalCategory.computeCompletionProposals(CompletionProposalCategory.java:340)
at org.eclipse.jdt.internal.ui.text.java.ContentAssistProcessor.collectProposals(ContentAssistProcessor.java:333)
at org.eclipse.jdt.internal.ui.text.java.ContentAssistProcessor.computeCompletionProposals(ContentAssistProcessor.java:289)
at org.eclipse.jface.text.contentassist.AsyncCompletionProposalPopup.lambda$10(AsyncCompletionProposalPopup.java:378)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:47)
at org.eclipse.jface.text.contentassist.AsyncCompletionProposalPopup.lambda$9(AsyncCompletionProposalPopup.java:377)
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1768)
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1760)
at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:387)
at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1312)
at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1843)
at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1808)
at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:188)
```
# System information
Eclipse SDK
Version: 2024-03 (4.31)
Build id: I20231212-0120

openjdk version "21" 2023-09-19 LTS
OpenJDK Runtime Environment Temurin-21+35 (build 21+35-LTS)
OpenJDK 64-Bit Server VM Temurin-21+35 (build 21+35-LTS, mixed mode, sharing)

Reproduce the issue with the Java constructor example and Content Assist while Java Postfix Template Proposals are enabled. Start at org.eclipse.jdt.internal.ui.text.java.PostfixCompletionProposalComputer.updateTemplateEngine and trace the stack through CompletionUnitStructureRequestor.createMethodHandle and Signature.createTypeSignature. Done means Content Assist completes without the reported exception in this scenario.

Written by the indexing model from the issue text.