Adding a simple line on code suddenly results in compilation error in GWT app? - java-8

Question

After just adding
String date = String.valueOf(personDTO.getBirthDate());
at a random .java, it no longer compiles.
I get this log after running mvn clean install:
at org.codehaus.mojo.gwt.shell.JavaCommand.execute (JavaCommand.java:327)
at org.codehaus.mojo.gwt.shell.CompileMojo.compile (CompileMojo.java:548)
at org.codehaus.mojo.gwt.shell.CompileMojo.doExecute (CompileMojo.java:415)
at org.codehaus.mojo.gwt.shell.AbstractGwtShellMojo.execute (AbstractGwtShellMojo.java:159)
at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:137)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:208)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:154)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:146)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:117)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:81)
at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build (SingleThreadedBuilder.java:56)
at org.apache.maven.lifecycle.internal.LifecycleStarter.execute (LifecycleStarter.java:128)
at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:305)
at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:192)
at org.apache.maven.DefaultMaven.execute (DefaultMaven.java:105)
at org.apache.maven.cli.MavenCli.execute (MavenCli.java:956)
at org.apache.maven.cli.MavenCli.doMain (MavenCli.java:290)
at org.apache.maven.cli.MavenCli.main (MavenCli.java:194)
at sun.reflect.NativeMethodAccessorImpl.invoke0 (Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke (DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke (Method.java:497)
at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced (Launcher.java:289)
at org.codehaus.plexus.classworlds.launcher.Launcher.launch (Launcher.java:229)
at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode (Launcher.java:415)
at org.codehaus.plexus.classworlds.launcher.Launcher.main (Launcher.java:356)`
It seems gwt plugin:
[ERROR] ]] failed with status 1
[ERROR] -> [Help 1]
org.apache.maven.lifecycle.LifecycleExecutionException:
Failed to execute goal org.codehaus.mojo:gwt-maven-plugin:2.7.0:compile
(gwtcompile) on project project-war: Command [[
Sorry if I cannot share the whole log with you for privacy issues :(
I am using Java 8.
Trying to mvn clean install GWT Project but plugin induces compiling failure.
I don't know what to do. I've tried reinstalling JDK, using OS terminal to compile instead of IntelliJ Maven Plugin, I've checked no imports are added with my simple new line, I've tried with different java types and avoiding String.valueOf() etc.
I have no idea how to read those logs or even how to exactly proceed. I wish to know the mental algorithm to solve this problem but all I see is a giant error trace.