[bug] Graphviz/DOT can't be outputted as ASCII art

[DrSensor]

Hi, I got converting from DOT to SVG working but not with the ASCII art.
Given this file 👇

@startuml
digraph foo {
  PlantUML -> Dot;
}
@enduml

Converting to ASCII art give me this error 👇 (though it works fine for SVG)

$ cat examples/test.dot | plantuml -txt -pipe

java.io.IOException: Stream closed
        at java.base/java.lang.ProcessBuilder$NullOutputStream.write(ProcessBuilder.java:442)
        at java.base/java.io.OutputStream.write(OutputStream.java:157)
        at java.base/java.io.BufferedOutputStream.flushBuffer(BufferedOutputStream.java:81)
        at java.base/java.io.BufferedOutputStream.flush(BufferedOutputStream.java:142)
        at java.base/java.io.FilterOutputStream.close(FilterOutputStream.java:182)
        at net.sourceforge.plantuml.cucadiagram.dot.ProcessRunner$MainThread.startThreads(ProcessRunner.java:185)
        at net.sourceforge.plantuml.cucadiagram.dot.ProcessRunner$MainThread.runJob(ProcessRunner.java:123)
        at net.sourceforge.plantuml.api.TimeoutExecutor$MyThread.run(TimeoutExecutor.java:79)
ProcessRunnerA java.io.IOException: Stream closed
java.io.IOException: Stream closed
        at java.base/java.io.BufferedInputStream.getBufIfOpen(BufferedInputStream.java:176)
        at java.base/java.io.BufferedInputStream.read(BufferedInputStream.java:275)
        at net.sourceforge.plantuml.cucadiagram.dot.ProcessRunner$ThreadStream.run(ProcessRunner.java:239)
Exception in thread "main" java.lang.UnsupportedOperationException
        at net.sourceforge.plantuml.ugraphic.UGraphicUtils.writeImage(UGraphicUtils.java:80)
        at net.sourceforge.plantuml.directdot.PSystemDot.exportDiagramNow(PSystemDot.java:89)
        at net.sourceforge.plantuml.AbstractPSystem.exportDiagram(AbstractPSystem.java:135)
        at net.sourceforge.plantuml.SourceStringReader.outputImage(SourceStringReader.java:154)
        at net.sourceforge.plantuml.Pipe.managePipe(Pipe.java:110)
        at net.sourceforge.plantuml.Run.managePipe(Run.java:359)
        at net.sourceforge.plantuml.Run.main(Run.java:166)

However, it works if it's in PlantUML format 🤔

@startuml
PlantUML --> Dot;
@enduml

$ cat examples/test.puml | plantuml -txt -pipe

,--------.
|PlantUML|
|--------|
|--------|
`--------'
     |    
     |    
  ,----.  
  |Dot;|  
  |----|  
  |----|  
  `----'  

Version

$ plantuml -version

PlantUML version 1.2019.07 (Tue Jun 25 02:59:20 WIB 2019)
(GPL source distribution)
Java Runtime: OpenJDK Runtime Environment
JVM: OpenJDK 64-Bit Server VM
Java Version: 12.0.1+12
Operating System: Linux
OS Version: 5.1.16-1-MANJARO
Default Encoding: UTF-8
Language: en
Country: US
Machine: wildan-pc
PLANTUML_LIMIT_SIZE: 4096
Processors: 4
Max Memory: 2,061,500,416
Total Memory: 130,023,424
Free Memory: 124,780,544
Used Memory: 5,242,880
Thread Active Count: 1

The environment variable GRAPHVIZ_DOT has not been set
Dot executable is /usr/bin/dot
Dot version: dot - graphviz version 2.40.1 (0)
Installation seems OK. File generation OK