Getting a NoClassDefFoundError when using functions of libraries with maven

Hey Guys, I am developing a plugin. I want to connect the plugin with firebase and use maven for this. As I heard I must use the maven-shade-plugin that the maven libraries are going to import into the .jar files. So my pom.xml looks like this:

Code:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>test.test_server_manager</groupId>
    <artifactId>server_manager</artifactId>
    <version>1.0</version>

    <dependencies>
        <dependency>
            <groupId>com.google.firebase</groupId>
            <artifactId>firebase-admin</artifactId>
            <version>6.6.0</version>
        </dependency>
    </dependencies>

    <build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-shade-plugin</artifactId>
            <version>2.1</version>
            <executions>
                <execution>
                    <phase>package</phase>
                    <goals>
                        <goal>shade</goal>
                    </goals>
                    <configuration>
                        <createDependencyReducedPom>false</createDependencyReducedPom>
                        <minimizeJar>true</minimizeJar>
                        <artifactSet>
                            <includes>
                                <include>mkremins:fanciful</include>
                                <include>org.json:json</include>
                            </includes>
                        </artifactSet>
                    </configuration>
                </execution>
            </executions>
        </plugin>
    </plugins>
    </build>

</project>

After adding that I already had the same error: NoClassDefFoundError. So how to fix this at all.
Thanks in advance.

Still looking for an answer...

EDIT by Moderator: merged posts, please use the edit button instead of double posting.