Hi, when I start my mod on the server, and try to place a block in the world, I get this error.

 

[21:55:23] [Server thread/FATAL] [minecraft/MinecraftServer]: Error executing task
java.util.concurrent.ExecutionException: java.lang.NoClassDefFoundError: net/minecraft/client/gui/inventory/GuiContainer
	at java.util.concurrent.FutureTask.report(Unknown Source) ~[?:1.8.0_212]
	at java.util.concurrent.FutureTask.get(Unknown Source) ~[?:1.8.0_212]
	at net.minecraft.util.Util.runTask(Util.java:54) [Util.class:?]
	at net.minecraft.server.MinecraftServer.updateTimeLightAndEntities(MinecraftServer.java:798) [MinecraftServer.class:?]
	at net.minecraft.server.dedicated.DedicatedServer.updateTimeLightAndEntities(DedicatedServer.java:415) [DedicatedServer.class:?]
	at net.minecraft.server.MinecraftServer.tick(MinecraftServer.java:743) [MinecraftServer.class:?]
	at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:592) [MinecraftServer.class:?]
	at java.lang.Thread.run(Unknown Source) [?:1.8.0_212]
Caused by: java.lang.NoClassDefFoundError: net/minecraft/client/gui/inventory/GuiContainer
	at com.edivad.solargeneration.blocks.SolarPanel.createNewTileEntity(SolarPanel.java:210) ~[SolarPanel.class:?]
	at net.minecraft.block.Block.createTileEntity(Block.java:1517) ~[Block.class:?]
	at net.minecraft.world.chunk.Chunk.createNewTileEntity(Chunk.java:839) ~[Chunk.class:?]
	at net.minecraft.world.chunk.Chunk.getTileEntity(Chunk.java:857) ~[Chunk.class:?]
	at net.minecraft.world.World.getTileEntity(World.java:2613) ~[World.class:?]
	at mcp.mobius.waila.network.MessageRequestTile$Handler.lambda$onMessage$0(MessageRequestTile.java:69) ~[MessageRequestTile$Handler.class:?]
	at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:1.8.0_212]
	at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:1.8.0_212]
	at net.minecraft.util.Util.runTask(Util.java:53) ~[Util.class:?]
	... 5 more
Caused by: java.lang.ClassNotFoundException: net.minecraft.client.gui.inventory.GuiContainer
	at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:191) ~[launchwrapper-1.12.jar:?]
	at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_212]
	at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_212]
	at com.edivad.solargeneration.blocks.SolarPanel.createNewTileEntity(SolarPanel.java:210) ~[SolarPanel.class:?]
	at net.minecraft.block.Block.createTileEntity(Block.java:1517) ~[Block.class:?]
	at net.minecraft.world.chunk.Chunk.createNewTileEntity(Chunk.java:839) ~[Chunk.class:?]
	at net.minecraft.world.chunk.Chunk.getTileEntity(Chunk.java:857) ~[Chunk.class:?]
	at net.minecraft.world.World.getTileEntity(World.java:2613) ~[World.class:?]
	at mcp.mobius.waila.network.MessageRequestTile$Handler.lambda$onMessage$0(MessageRequestTile.java:69) ~[MessageRequestTile$Handler.class:?]
	at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:1.8.0_212]
	at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:1.8.0_212]
	at net.minecraft.util.Util.runTask(Util.java:53) ~[Util.class:?]
	... 5 more
Caused by: net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerException: Exception in class transformer net.minecraftforge.fml.common.asm.transformers.SideTransformer@5c82cd4f from coremod FMLCorePlugin
	at net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerWrapper.transform(ASMTransformerWrapper.java:260) ~[forgeSrc-1.12.2-14.23.5.2838.jar:?]
	at net.minecraft.launchwrapper.LaunchClassLoader.runTransformers(LaunchClassLoader.java:279) ~[launchwrapper-1.12.jar:?]
	at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:176) ~[launchwrapper-1.12.jar:?]
	at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_212]
	at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_212]
	at com.edivad.solargeneration.blocks.SolarPanel.createNewTileEntity(SolarPanel.java:210) ~[SolarPanel.class:?]
	at net.minecraft.block.Block.createTileEntity(Block.java:1517) ~[Block.class:?]
	at net.minecraft.world.chunk.Chunk.createNewTileEntity(Chunk.java:839) ~[Chunk.class:?]
	at net.minecraft.world.chunk.Chunk.getTileEntity(Chunk.java:857) ~[Chunk.class:?]
	at net.minecraft.world.World.getTileEntity(World.java:2613) ~[World.class:?]
	at mcp.mobius.waila.network.MessageRequestTile$Handler.lambda$onMessage$0(MessageRequestTile.java:69) ~[MessageRequestTile$Handler.class:?]
	at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:1.8.0_212]
	at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:1.8.0_212]
	at net.minecraft.util.Util.runTask(Util.java:53) ~[Util.class:?]
	... 5 more
Caused by: java.lang.RuntimeException: Attempted to load class net/minecraft/client/gui/inventory/GuiContainer for invalid side SERVER
	at net.minecraftforge.fml.common.asm.transformers.SideTransformer.transform(SideTransformer.java:62) ~[forgeSrc-1.12.2-14.23.5.2838.jar:?]
	at net.minecraftforge.fml.common.asm.ASMTransformerWrapper$TransformerWrapper.transform(ASMTransformerWrapper.java:256) ~[forgeSrc-1.12.2-14.23.5.2838.jar:?]
	at net.minecraft.launchwrapper.LaunchClassLoader.runTransformers(LaunchClassLoader.java:279) ~[launchwrapper-1.12.jar:?]
	at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:176) ~[launchwrapper-1.12.jar:?]
	at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_212]
	at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_212]
	at com.edivad.solargeneration.blocks.SolarPanel.createNewTileEntity(SolarPanel.java:210) ~[SolarPanel.class:?]
	at net.minecraft.block.Block.createTileEntity(Block.java:1517) ~[Block.class:?]
	at net.minecraft.world.chunk.Chunk.createNewTileEntity(Chunk.java:839) ~[Chunk.class:?]
	at net.minecraft.world.chunk.Chunk.getTileEntity(Chunk.java:857) ~[Chunk.class:?]
	at net.minecraft.world.World.getTileEntity(World.java:2613) ~[World.class:?]
	at mcp.mobius.waila.network.MessageRequestTile$Handler.lambda$onMessage$0(MessageRequestTile.java:69) ~[MessageRequestTile$Handler.class:?]
	at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:1.8.0_212]
	at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:1.8.0_212]
	at net.minecraft.util.Util.runTask(Util.java:53) ~[Util.class:?]
	... 5 more

This is the project repository: https://github.com/Edivad99/SolarGeneration

Does anyone know how to solve this problem, because I'm really freaking out

Thanks for answering me, I can't understand what you mean by point 1. I don't know how to fix point 4, could you help me out?