Eclipse has an internal error during Launching

I am facing this issue with eclipse (Its eclipse Oxygen) I am unable to run a simple Java program
This happen after I modified the project to be a Maven project.
what I tried so far:
I updated the software using (help > check for updates)
deleted the Workspace, and created a new one.

!ENTRY org.eclipse.core.jobs 4 2 2017-12-21 13:15:48.670
!MESSAGE An internal error occurred during: "Launching MailProcessor".
!STACK 0
java.lang.NullPointerException
    at org.eclipse.jdt.launching.JavaRuntime.isModularJava(JavaRuntime.java:1043)
    at org.eclipse.jdt.launching.JavaRuntime.isModularConfiguration(JavaRuntime.java:1020)
    at org.eclipse.jdt.launching.JavaRuntime.computeJREEntry(JavaRuntime.java:2341)
    at org.eclipse.m2e.jdt.internal.launch.MavenRuntimeClasspathProvider.computeUnresolvedClasspath(MavenRuntimeClasspathProvider.java:93)
    at org.eclipse.jdt.internal.launching.RuntimeClasspathProvider.computeUnresolvedClasspath(RuntimeClasspathProvider.java:57)
    at org.eclipse.jdt.launching.JavaRuntime.computeUnresolvedRuntimeClasspath(JavaRuntime.java:1509)
    at org.eclipse.jdt.launching.AbstractJavaLaunchConfigurationDelegate.getBootpathExt(AbstractJavaLaunchConfigurationDelegate.java:299)
    at org.eclipse.jdt.launching.AbstractJavaLaunchConfigurationDelegate.getVMSpecificAttributesMap(AbstractJavaLaunchConfigurationDelegate.java:615)
    at org.eclipse.jdt.launching.JavaLaunchDelegate._OT$callOrig(JavaLaunchDelegate.java:71)
    at org.objectteams.Team._OT$terminalCallNext(Team.java:598)
    at org.objectteams.Team._OT$callNext(Team.java:582)
    at org.eclipse.objectteams.otdt.internal.debug.adaptor.launching.JDTLaunchingAdaptor._OT$callNext(JDTLaunchingAdaptor.java:56)
    at org.eclipse.objectteams.otdt.internal.debug.adaptor.launching.JDTLaunchingAdaptor._OT$callReplace(JDTLaunchingAdaptor.java:56)
    at org.objectteams.Team._OT$callAllBindings(Team.java:557)
    at org.eclipse.jdt.launching.JavaLaunchDelegate.callAllBindings(JavaLaunchDelegate.java)
    at org.eclipse.jdt.launching.JavaLaunchDelegate.launch(JavaLaunchDelegate.java)
    at org.eclipse.debug.internal.core.LaunchConfiguration.launch(LaunchConfiguration.java:885)
    at org.eclipse.debug.internal.core.LaunchConfiguration.launch(LaunchConfiguration.java:739)
    at org.eclipse.debug.internal.ui.DebugUIPlugin.buildAndLaunch(DebugUIPlugin.java:1039)
    at org.eclipse.debug.internal.ui.DebugUIPlugin$8.run(DebugUIPlugin.java:1256)
    at org.eclipse.core.internal.jobs.Worker.run(Worker.java:56)

I faced a similar issue in the past, you can try the following:

1. Exit Eclipse
2. Go to your workspace folder in the file explorer
3. Look for the .metadata folder and delete it
4. Re-open Eclipse

Hope it works.