Exceptions are thrown when trying to generate tests for abstract classes

[volivan239]

Description

Currently, if you pass methods from abstract classes to tests generator, you are likely to have an uncaught exception that will ruin test generation

To Reproduce

Launch the plugin on a class C in this example:

abstract class C {
    abstract public int f();
}

class Cimpl extends C {
    @Override
    public int f() { return 0; }
}

Expected behavior

No tests are generated and C.f() is not even proposed for testing in dialog.

Actual behavior

Plugin shows a dialog with C.f() method. After launching plugin fails with IllegalArgumentException.

Visual proofs (screenshots, logs, images)

java.lang.IllegalArgumentException: Can't retrieve body for <rndpkg.C: int f()>
	at org.utbot.engine.ExtensionsKt.jimpleBody(Extensions.kt:114)
	at org.utbot.framework.util.SootUtilsKt.jimpleBody(SootUtils.kt:102)
	at org.utbot.engine.UtBotSymbolicEngine.<init>(UtBotSymbolicEngine.kt:157)
	at org.utbot.framework.plugin.api.TestCaseGenerator.createSymbolicEngine(TestCaseGenerator.kt:278)
	at org.utbot.framework.plugin.api.TestCaseGenerator.access$createSymbolicEngine(TestCaseGenerator.kt:51)
	at org.utbot.framework.plugin.api.TestCaseGenerator$generate$4$1$1.invokeSuspend(TestCaseGenerator.kt:166)
	at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
	at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:106)
	at kotlinx.coroutines.EventLoopImplBase.processNextEvent(EventLoop.common.kt:274)
	at kotlinx.coroutines.BlockingCoroutine.joinBlocking(Builders.kt:84)
	at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking(Builders.kt:59)
	at kotlinx.coroutines.BuildersKt.runBlocking(Unknown Source)
	at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking$default(Builders.kt:38)
	at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source)
	at org.utbot.common.ConcurrencyKt.runBlockingWithCancellationPredicate(Concurrency.kt:38)
	at org.utbot.framework.plugin.api.TestCaseGenerator$generate$4.invoke(TestCaseGenerator.kt:158)
	at org.utbot.framework.plugin.api.TestCaseGenerator$generate$4.invoke(TestCaseGenerator.kt:51)
	at org.utbot.common.ConcurrencyKt.runIgnoringCancellationException(Concurrency.kt:47)
	at org.utbot.framework.plugin.api.TestCaseGenerator.generate(TestCaseGenerator.kt:157)
	at org.utbot.framework.plugin.api.TestCaseGenerator.generate$default(TestCaseGenerator.kt:144)
	at org.utbot.intellij.plugin.generator.UtTestsDialogProcessor$createTests$2$1.run(UtTestsDialogProcessor.kt:196)
	at com.intellij.openapi.progress.impl.CoreProgressManager$TaskRunnable.run(CoreProgressManager.java:935)
	at com.intellij.openapi.progress.impl.CoreProgressManager.lambda$runProcessWithProgressAsync$5(CoreProgressManager.java:442)
	at com.intellij.openapi.progress.impl.ProgressRunner.lambda$submit$3(ProgressRunner.java:235)
	at com.intellij.openapi.progress.impl.CoreProgressManager.lambda$runProcess$2(CoreProgressManager.java:170)
	at com.intellij.openapi.progress.impl.CoreProgressManager.registerIndicatorAndRun(CoreProgressManager.java:629)
	at com.intellij.openapi.progress.impl.CoreProgressManager.executeProcessUnderProgress(CoreProgressManager.java:581)
	at com.intellij.openapi.progress.impl.ProgressManagerImpl.executeProcessUnderProgress(ProgressManagerImpl.java:60)
	at com.intellij.openapi.progress.impl.CoreProgressManager.runProcess(CoreProgressManager.java:157)
	at com.intellij.openapi.progress.impl.ProgressRunner.lambda$submit$4(ProgressRunner.java:235)
	at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1700)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.util.concurrent.Executors$PrivilegedThreadFactory$1$1.run(Executors.java:668)
	at java.base/java.util.concurrent.Executors$PrivilegedThreadFactory$1$1.run(Executors.java:665)
	at java.base/java.security.AccessController.doPrivileged(Native Method)
	at java.base/java.util.concurrent.Executors$PrivilegedThreadFactory$1.run(Executors.java:665)
	at java.base/java.lang.Thread.run(Thread.java:834)

Environment

Generate for Java, JUnit5 with Mockito static mocking.

Additional context

Similar problems happen when you try to generate tests for abstract classes that has no implementations.

See also #221.