NPE in launcher wizard if the ale key points to the wrong ale file

if the dsl file points to a non existing ale file
then in the launch configuration, trying to browse for the main method raises a NullPointerException

```
java.lang.NullPointerException
at org.eclipse.emf.ecoretools.ale.ide.WorkbenchDsl.pluginToFile(WorkbenchDsl.java:96)
at org.eclipse.emf.ecoretools.ale.ide.WorkbenchDsl.convertToFile(WorkbenchDsl.java:66)
at org.eclipse.emf.ecoretools.ale.ide.WorkbenchDsl.lambda$0(WorkbenchDsl.java:50)
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)
at java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:580)
at org.eclipse.emf.ecoretools.ale.ide.WorkbenchDsl.resolveUris(WorkbenchDsl.java:50)
at org.eclipse.emf.ecoretools.ale.ide.WorkbenchDsl.(WorkbenchDsl.java:33)
at org.eclipse.gemoc.ale.interpreted.engine.Helper.gemocDslToAleDsl(Helper.java:24)
at org.eclipse.gemoc.ale.interpreted.engine.ui.SelectMainMethodDialog.update(SelectMainMethodDialog.java:48)
at org.eclipse.gemoc.ale.interpreted.engine.ui.SelectMainMethodDialog.(SelectMainMethodDialog.java:34)
at org.eclipse.gemoc.ale.interpreted.engine.ui.launcher.tabs.LaunchConfigurationMainTab$8.widgetSelected(LaunchConfigurationMainTab.java:346)
at org.eclipse.swt.widgets.TypedListener.handleEvent(TypedListener.java:249)

```

Instead, it should raise a proper error message in the eclipse log

Steps to reproduce: get the official example, change the dsl file and introduce a typo in the path for the ale file. in the modeling workbench create a new launch configuration and try to browse the main method for the language