Consistent SpotBugs configuration & warning fixes.

platform-independent/java/gui/build.gradle

@@ -1,7 +1,6 @@
 plugins {
     id 'java'
     id 'application'
-    id 'com.github.spotbugs' version '2.0.0'
     id 'com.github.johnrengelman.shadow' version '5.1.0'
 }
 
@@ -13,6 +12,7 @@ dependencies {
     implementation group: 'ch.qos.logback', name: 'logback-classic', version: '1.1.2'
     implementation group: 'com.yuvimasory', name: 'orange-extensions', version: '1.3.0'
     implementation group: 'com.github.tulskiy', name: 'jkeymaster', version: '1.2'
+    implementation group: 'com.github.spotbugs', name: 'spotbugs-annotations', version: '4.0.0-beta4'
 
     compile project( ':masterpassword-model' )
 }